Abstract:
Embodiments of a method and apparatus are described which provide for a consistent, continuous and/or repeating signal. Such a repeating signal may be used to set the controls for a processor. For example, an embodiment of a preview sampler described herein allows a user to repeatedly output a data segment into a processor so that the processor can be adjusted to achieve a desired effect. Such a method and apparatus provides improved results when compared to an individual attempting to repeatedly generate a data signal (e.g., a note or chord on a musical instrument).

Description:
BACKGROUND OF THE INVENTION 
     The present invention pertains to a process for providing a consistent, continuous and/or repeating signal. More particularly, the present invention pertains to a method and apparatus for creating a continuous/repeating signal/data stream from an original source signal/data stream, and providing this signal/data stream continuously/repeatedly. 
     It is known in a variety of arts to use devices known as processors to modify signals from a variety and multiplicity of sources. The processors themselves have varying numbers of controllable parameters, with varying degrees of complexity in the setting of each parameter. Generally, a human operator, or constructed control device, makes these adjustments on the basis of some perception of the results of adjustments as they are made. A common occurrence in the audio field supplies a simple example. In a situation where it is desired to amplify or record a musical instrument, an electronic signal (or digital or other useful signal) is usually presented via a microphone (or other sound transducer) and its associated amplifier. Typically, this signal is then modified through one or several processors such as equalizers, filters, compressors, reverberators, and many other effects devices. A musician will play the musical instrument repeatedly, so a sound engineer can listen to changes in the sound produced by the processor(s) as he varies each control parameter of each processor. Under these circumstances, three problems arise while modifying the signal: 
     1 The listener must hear ONLY the electronic sound being modified, so the original acoustic sound must be isolated from the listener. This is normally accomplished in a recording studio by having separate, acoustically isolated rooms for playing and listening or by recording the instrument (onto tape, etc.) and then using the recording as the sound source. 
     Some headphones provide a limited degree of isolation, and are used when isolation from the acoustic sound is impossible (usually in live performance situations) or unaffordable. 
     2 The performer must play a variety of short phrases over and over, so that the listener can hear the effects of the equipment being used. (With musical material varying, it is difficult to judge whether a change is due to a knob turned or a note played more loudly or differently). This process can be very draining on a performer, as making adjustments carefully enough to get a good sound for either recording or live playing can take a lot of time. Also, many performers are not good at playing a phrase consistently, which makes the listener/engineer&#39;s job difficult or impossible. 
     3 If the performer and listener are the same, and a recording system is not available, the only recourse available (beyond just guessing) is to use headphones, with the limited isolation mentioned above. For loud instruments (e.g., drums), there is no headphone that provides enough isolation to do a good job. For a singer, headphones do not isolate at all, because there is an internal sound transmission through the singer&#39;s body. 
     These problems are not pertinent in situations where the signal does not require acoustic isolation, but other problems may arise. For example, a signal source may have a degree of randomness that makes the adjustment of a processor parameter difficult, although the processor will be able to accomplish its goal once properly set. 
     SUMMARY OF THE INVENTION 
     These and other problems are addressed by a method and apparatus of the present invention. According to an embodiment of the present invention, a system can be customized to take a conveniently small test signal (such as a short musical phrase for the example above), record it on a suitable medium, and play it repeatedly into any desired processor or other system. In one embodiment of the present invention, standard components can be used to build a device where: 
     1. A short length, limited use sampler (or other recording medium) of adequate quality is set into READY MODE, 
     2. Upon an operator&#39;s signal, or upon detecting the desired signal at a pre-selected level (a “threshold”), an appropriate length (e.g., about 1 or 2 seconds) of sound or other signal is recorded, and 
     3. The recorded signal can be immediately played continuously (i.e. “looped”), allowing the operator to make the necessary adjustments to the processor(s). 
     Specifically, for the acoustic example above, some of the advantages of the system are: 
     1. There is no need to record a long section of signal, and no other recording equipment is needed. 
     2. There is no need for an isolated room. 
     3. Where the signal is generated by a musician or other person, the performer does not have to play test phrases repetitively, avoiding mental and physical fatigue. Under many circumstances, even a non-musician can generate a phrase well enough so that the performer is not needed for processor adjustments at all. Also, the ‘perfect’ consistency of a repeated loop is often the ideal signal for adjustments of this type, and is therefor often a better test signal than even a good player may provide. 
     4. Where the signal is generated by a musician or other person, a recorded version of the signal may be the only possible way to allow that same person to be the operator making adjustments (that is, for the performer to be directly involved in the sound control process). This is particularly true for a singer, who can never be isolated from his/her voice. A recorded signal is the only known solution. In live performance situations, recording may not be feasible or available. The present invention allows a singer to control the process of creating the “sound” that his/her voice will make through the sound system that the audience will be listening to. 
     5. For any specific circumstance or set of circumstances, experimentation can be used to optimize the length and speed of repetitions to allow quickest adjustment and minimum fatigue in listening. 
     According to the present invention, this process can be accomplished using a commercially available sampler. This is cumbersome, as it requires patching or switching the sampler into some pre-processor point in the signal path, and requires several steps in the sampler&#39;s operation, such as RECORD, TRIM SAMPLE, and SET LOOP LENGTH. 
     In some circumstances, two (or more) signals may have interactions that require adjustments to be made interactively. One such circumstance is when dealing with acoustic signals, where there is often more than one sound source in use at a time. For example, there may be two instruments playing in close proximaty, such as a violin and a piano. Also, some musical instruments often require the use of more than one microphone (common examples are a piano and a drum set). Each microphone is placed to pick up only a particular instrument or part of an instrument. In practice, the sound from other instruments or unwanted regions of the same instrument “crosses” into all microphones. This is called acoustic crosstalk. The sound of an instrument&#39;s crosstalk into other microphones sometimes approaches or even surpasses the level of that instrument&#39;s sound in its own microphone. Thus, when modifying a single microphone&#39;s signal, it is important to be able to hear the microphone&#39;s signal mixed with any other pertinent microphone signal, as well as alone, so that the final product is as desired. A two channel version of the system, one channel for each signal, accomplishes this. 
     The problems as described for two source signals are the same for more than two source signals. The solution is to have one channel for each signal. For circumstances where this is not practical, a method is provided to accommodate multiple signal channels with the use of only two sampler channels; channel A for the signal being modified, and channel B for a mix of all other pertinent signals as they will be perceived in relation to channel A. For the acoustic example described above, by turning channel B off and on, the listener can switch back and forth between the single microphone alone (where it is easiest to hear how the sound is being affected) and in combination with all the other microphones, (which will be the final product required). This embodiment is presented below, along with a switching mechanism to facilitate use with a multiplicity of signals that need to be adjusted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general block diagram of an apparatus constructed according to an embodiment of the present invention. 
     FIG. 2 is a detailed block diagram of the apparatus of FIG. 1, shown as a two channel device. 
     FIG. 3 is a general block diagram of the apparatus of FIG. 1, but constructed as a multi-channel device. 
     FIG. 4 is a multiple channel block diagram of the apparatus of FIG. 3, using parallel microprocessors. 
     FIG. 5 is a multiple channel block diagram of the apparatus of FIG. 3, using a single microprocessor. 
     FIG. 6 is a detailed view of part of the apparatus of FIG.  5 . 
     FIG. 7 is a general block diagram of an embodiment that allows a two channel device to substitute for the multi-channel embodiment of FIG.  3 . 
     FIG. 8 is a general block diagram of an embodiment which incorporates a multi-purpose device for the preview sampler  12  of the apparatus of FIGS. 1 and 3. 
     FIG. 9 is a general block diagram of an apparatus that employs various aspects of the method of the present invention. 
     FIG. 10 is a detailed schematic diagram of an implementation of FIG.  9 . 
     FIG. 11 is a block diagram of an embodiment of the apparatus of FIGS. 9 and 10, incorporated into a signal mixer. 
     FIG. 12 a  is a detailed schematic diagram of the apparatus of FIG.  11 . 
     FIG. 12 b  shows the pin-out arrangement for a switch used in FIG. 12 a  and FIG.  13 . 
     FIG. 13 is a schematic diagram of an embodiment of the apparatus of FIGS. 9 and 10, designed as a stand-alone unit to be interfaced with a separate, pre-existing signal mixer. 
     FIG. 14 is a schematic diagram of one possible implementation of a selection control device to be used with the apparati of FIGS.  11 - 13 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a block diagram of an apparatus which facilitates the tuning of a signal in a processor or processors is shown. An appropriate length of a source signal  10  is fed to and recorded by the preview sampler  12 . The preview sampler then repeatedly plays the recorded signal and supplies it to the input of the signal processor(s)  13 , whose output may be supplied to an output device  19  such as a loudspeaker system or television screen. Using the example of audio frequencies (approximately between 20 Hz and 20 kHz), this component can be constructed by using a typical digital audio sampler, such as a Kurzweil K-2000, as the preview sampler  12 . The following description is written at a level for a user skilled in the use of the K-2000 specifically, though one generally skilled in the art of the modern sampler use will understand it. Aside from showing an implementation of the method using a general purpose device, the example also shows how cumbersome it is to use such a general purpose device for the method, as opposed to an embodiment of the present invention specifically designed for the purpose (whose description immediately follows this example). The signal ( 10 ) output is easily coupled into the input of the sampler, and the output of the sampler is connected into the input of the signal processor(s)  13 . Input and output levels are set, as is generally known in the art. Through a complex operation one could operate the sampler as follows, described for the example of the Kurzweil K 2000 (words in capital letters are either buttons to press, “soft buttons” to press, or parameters to set): 
     1. Set the sampler into record ready mode, set to start recording upon the signal&#39;s crossing a threshold: 
     A-Press [MASTER/SAMPLE] 
     B-Set these parameters (the signal must be present to the sampler when setting the GAIN and THRESHOLD parameters): 
     [SAMPLE]=Set this value to none. 
     [INPUT]=Analog 
     [TIME](length)—is only available in whole unit seconds 
     [MONITOR] to [ON] if needed 
     [GAIN] as is suitable (e.g. at 0) 
     [RATE] to 44.1 kHz 
     [MODE] to 1 or 2 channels (max available are 2) 
     [THRESHOLD] to an amount somewhat less than the signal appears in the sampler&#39;s meters. 
     2. Send the signal (e.g., play an instrument) and record it into the sampler&#39;s memory: 
     C-Press [AUTO], then send the signal/play the instrument. 
     D-the sample number that has been assigned should be noted, which is needed to edit in step 3, and to eventually erase when done. 
     3. Set the sampler into sample edit mode, in order to trim the length of the sample to a useful length: 
     E-Select [EDIT] (you&#39;re still in [MASTER/SAMPLE] mode, with the sample number unchanged). 
     F-Hold down the C64 note on the keyboard (a piece of adhesive tape may be used for this purpose). 
     G-Move to [L] (=loop) parameter with arrow button. 
     H-Set the [L] parameter to the start point (0.000). 
     I-Move to [E] (=end) parameter with arrow button. 
     J-Set the [E] parameter, so that the length of the loop is convenient (depends on material—my experience shows a typical repeat speed would be about ¾ second for a single note or chord from a musical instrument). 
     4. Set the sampler into play looped mode (looped play means continuous replay, with no time between each replay). 
     K-continue to hold down the C 64 note on the keyboard or 
     1-Save the sample as set for length above, 
     [EXIT]/[YES]/[REPLACE] 
     2-Then create a one measure song in the sampler&#39;s song mode, setting the tempo that is convenient for tuning. This process includes the following steps: 
     [SONG]/set [PROGRAM] to the number above/[MISC]/[RECMODE]=Linear/[PLAYMODE]=Loop/[COUNTOFF]=1/[MAIN]/arrow to [TEMPO] &amp; set to 80, for example/set [RECTRK] to 1/[RECORD]/[PAUSE] then after countdown bar, evenly play four beats, then press [STOP]/[YES]/[REPLACE]/[PLAY]/adjusting [TEMPO] as needed. 
     5. Start tuning the processor(s)  13 . 
     6. Repeat the above for each signal sample. 
     7. When done, erase these samples and songs from memory. A few steps under [MASTER/SAMPLE] and perhaps [SONG] are needed to do this. 
     The above explanation requires some knowledge of the K2000 (e.g., knowing which controls affect which parameters). Each brand and model of sampler operates differently, and instructions for any other model would thus be different, though similar in nature and scope. The K2000 is but one example and one skilled in the art will appreciate that another apparatus can be used as preview sampler  12 . 
     Turning now to an example of a preview sampler  12  constructed according to an embodiment of the present invention, the connections are the same as above. Signal  10 &#39;s output is connected to the input of the preview sampler  12 , and the output of the preview sampler is connected to the signal processor(s)  13 . Levels are set, as is generally known in the art. This embodiment requires only two controls: a Record Ready button, and a Loop Length setting control. The sequence of over 20 actions described in #s 1-7 above are replaced by the following: 
     1. (OPTIONAL) Adjust loop length control knob if preset length is not suitable. 
     2. Set a trigger threshold. 
     3. Press the record ready button. 
     4. Send the signal (e.g., play an instrument). 
     5. Start tuning the processor(s)  13 . 
     6. Repeat the above for each signal sample. 
     Alternatively/additionally, the loop length can be adjusted during the tuning process, as desired. Input and output gain controls may be added where desirable. The recording process begins at the crossing of a threshold by default, but an option to begin immediately upon an operator control signal is made available. One example of a threshold detector, commonly known in the art, is seen within FIG. 10, element  92 . 
     FIG. 2 shows one embodiment of this design. As audio frequency codices commonly come as dual channel designs (to accommodate stereo signals for the consumer market), this example is shown with two channels. One channel or multichannel devices may also be used. User Interface  23  may include a status display; ‘Record Ready’, ‘Stop’, ‘Sample Length’, ‘Tempo’ and ‘Trigger Threshold’ controls. Some devices may permit the use of Data Bus  24  for the input from and output to User Interface  23 , instead of the separate I/O port as shown. 
     The operator sets a trigger threshold, and then sends a control signal from user interface  23  to microprocessor  21  to begin the record/play sequence. Source signal/s  10  are fed into the signal input/s of the Codec  20 , which performs an analog-to-digital conversion (and also, later in the signal flow, a digital-to-analog conversion). The digitized signal is sent out codec  20 &#39;s Serial Data Output (SDO) to the Receive Data  0  port (RXD 0 ) of Microprocessor  21 , which writes the data to Memory  22  via the microprocessor&#39;s Address Bus. When recording to memory is complete, the Memory  22 &#39;s contents are continuously read via the microprocessor&#39;s Data Bus  24 , and sent out microprocessor  21 &#39;s Transmit Data  0  port (TXD 0 ) to the Serial Data Input (SDI) of Codec  20 . Codec  20  converts the received data into analog signal/s, and presents them at its signal outputs. The signal/s may then be sent to the Signal Processor(s)  13 . 
     FIG. 3 shows a multi-channel version of the embodiment in FIG. 1, for situations where several simultaneous signals  10   a,b , . . .  10   n  require individual processing. Each channel is identical to the single channel of FIG. 1, but they all operate simultaneously and are simultaneously controlled by a single set of controls. This is particularly helpful where the outputs of processors  13   a,b, . . . n  interact, and may therefore affect the adjustment of each other&#39;s processor settings. A common example is where the outputs of a variety of audio signals will be combined, as when the output of several microphones placed near one or more musical instruments or voices are mixed to the two channels suitable for a common stereo. In these situations, there is likely to be a significant amount of acoustic crosstalk found in the microphone signals. The embodiment may include as many channels as are needed (or affordable) for a given situation. Once the levels have been set for each signal path, the operation of the preview sampler is the same as described above for the embodiment of FIG. 1, regardless of how many channels are involved. This embodiment allows processor adjustments for different sources to be made concurrently, and with a single sample recording. 
     Implementation is possible with inexpensive, relatively slow microprocessors, as all that is required is a simple read/write instruction cycle. However, for signals that have a significant bandwidth, device limitations need to be accounted for. As the number of signals to be sampled increases, the processor needs to be faster and/or able to handle multiple inputs, as well as able to address larger data memory blocks. Also, all the devices must be able to read and write at an appropriate rate. Consider the relatively small bandwidth (about 20 kHz) of this audio example: for the commonly accepted audio standard of 16 bit resolution at 44.1 kHz, the microprocessor must run somewhat in excess of 0.7056 MHz, multiplied by the number of signals. One way around this ever increasing burden on the components (especially the microprocessor) is the use of multiple sets of (codecs+microprocessors+memory) running in parallel, one set for each signal or pair of signals. 
     FIG. 4 is one implementation of the embodiment of FIG. 3, using parallel sets of the type shown in FIG.  2 . The function and operation is the same as for FIG. 2, except that the user interface  23  interfaces with all the microprocessors  21 . In this example, the codecs  20  shown are 2 channels each, hence the total of 2n possible signals, rather than just n. The number of these sets of codecs+microprocessors+memory may be expanded to the limits of the fanout of a common clock and user interface. 
     FIG. 5 shows a second implementation of the embodiment of FIG. 3, in this case using a single microprocessor instead of a group of parallel microprocessors. Integrating the digital signal inputs to a single microprocessor  21  may provide advantages, including cost and a flexibility for other uses and/or configurations: one example is a mode wherein a single channel at a time is sampled for independent playback, rather than recording all channels at once, which allows a longer sample time for a fixed amount of memory. For reasons stated above, this example uses two channel codecs, which expands the total of possible signals from n to 2n. 
     The interface, function and operation are the same as in FIG.  2 . The operator sets a trigger threshold, and then sends a control signal from user interface  23  to microprocessor  21  to begin the record/play sequence. Source signals 1-2n  10  are fed into the signal inputs of the Codecs  20 , which perform analog-to-digital conversions (and also, later in the signal flow, digital-to-analog conversions). The digitized signals are sent out the codec  20  Serial Data Outputs (SDOs) to the Receive Data  0  port (RXD 0 ) of Microprocessor  21 , which writes the data to Memory  22  via the microprocessor&#39;s Address Bus. When recording to memory is complete, the Memory  22 &#39;s contents are continuously read via the microprocessor&#39;s Data Bus  24 , and sent out microprocessor  21 &#39;s Transmit Data  0  port (TXD 0 ) to the Serial Data Inputs (SDIs) of Codecs  20 . Codecs  20  convert the received data into analog signals, and present them at their signal outputs. The signals may then be sent to the Signal Processors  13 . 
     Referring to FIG. 6, a possible arrangement for the device of FIG. 5 uses Analog Devices&#39; AD1847 CODEC ADC/DAC dual-channel devices. For an eight-channel sampler, four daisy-chained Codecs  20  (three are shown) may be interfaced with the low-end Intel 80C186 microprocessor  51  through one or two serial ports. The microprocessor can directly address 1 MegaByte of RAM and can refresh dynamic memory. Utilizing its 24-bit data bus, the effective direct memory capacity is 3 Mbytes. For longer record times, the CODECs can be configured for 8-bit data conversion, either linearly or with a companding algorithm. Audio dynamic range is reduced, but total record time available is twice that of 16-bit data storage. Alternatively, other addressing schemes or a microprocessor with a larger address bus may be used where storage requirements are greater than the limits of this particular device. 
     FIG. 7 shows a block diagram of a method for imitating the capabilities of a multi-channel preview sampler by combining a one or two channel version with a switching network. Here, a two channel version of the embodiment in FIG. 1 is combined with a switching and mixing matrix to allow the adjustment of the processors for a plurality of source signals. This embodiment only allows the tuning of one source signal at a time, and a new sample must be recorded for each change of signal to be tuned. It is therefore not as convenient as the multi-channel embodiment of FIG. 3, but may be cheaper and/or more flexible in some situations. A two-channel embodiment of the preview sampler of FIG. 1 (e.g., as in FIG. 2) can be used as a stand alone device by cabling the inputs and outputs to insert it appropriately within the signal path of a multi-channel signal mixer. However, the sampling and tuning process is significantly easier to use when integrated with a multi-channel signal mixer and automatic signal switching system. 
     One of the signals  10  is selected to be adjusted, designated the Solo Signal. A switching system (not shown here, but discussed with FIGS. 9-14) simultaneously affects the three Solo Select Switches  71 ,  74  and  75 . Switch  71  removes the output of the Solo Signal from its normal path to its processor(s), and redirects it to channel A of preview sampler  12 . Switch  74  connects the channel A output of preview sampler  12  to the input of the signal processor(s) that pertain to the selected solo channel. Switch  75  directs the output of the selected solo channel&#39;s processor(s) to output device  19   a.    
     All other signals are passed directly to their designated processor(s), whose outputs are sent to signal mixer  76 . This mixer is set to maintain the ratios of signal strength, in balance with the output at the selected solo channel&#39;s processor(s) output, that will be used in the final mix (after processor adjustment is completed, and the preview sampler is effectively removed from the environment). The mixer&#39;s output is directed to channel B of the preview sampler  12 . The output of this channel B may then be directed to output device  19   b . Note that, as the channel B recording is of a mix of signals after they have passed through their processor(s), adjustments made to any processor(s) that go to channel B of sampler  12  will not affect the signal sent to output device  19   b . Thus, a new recording must be made each time a different signal&#39;s processor(s) will require adjustment. 
     In this embodiment, the output of a selected ‘Solo’ signal is available via its processor(s) at  19   a , and the sum of all other signals present is given at  19   b . This allows the adjustment of each and/or all signal processor(s), sequentially, allowing the adjustments to made with or without the influence of the total signal field. Where each signal channel is completely independent, there is no need for the mix of other channels. In this case, a single channel version of Preview Sampler (FIG. 1) may be used, and items  10   b ,  76 , and  19   b  would be eliminated from FIG.  7 . 
     FIG. 8 shows a general block diagram for the use of a common, pre-existing reverberation or effects unit adapted/modified for use as a one or two channel sampler unit in addition to its normal function. This enables an embodiment of the present invention with a minimum of cost and effort. These devices are commonly available as two channel units, so this example is described for two channels, which adds functionality to Selections 2 and 3 below. Where desired, a single channel is implemented similarly. Almost any digital stereo reverb unit can be modified to function as such a preview sampler. The actual modifications are described below; seen here is how the unit functions in its environment. 
     Three possible and likely arrangements, which may easily be accomplished by a function select switch, are: 
     Selection 1) The unit operates as it normally does, set for an effect processing program, such as reverberation. It can be situated in a two channel insert point in a signal mixer&#39;s auxiliary send/return loop. The signal to Effect Unit  82 &#39;s inputs A and B are from auxiliary send outputs  81  left and right (or mono) of a common signal mixer. Outputs A and B of Effects Unit  82  are directed to the auxiliary return inputs  83  left and right of the common mixer. 
     Selection 2) The unit is set to sampler mode, (described below). The signal to Effect Unit  82 &#39;s inputs A and B now come from two independent signal sources  81 , such as the amplified signals x and y from two microphones set to record a piano. Outputs A and B of Effects Unit  82  are then directed to the inputs  83  of the signal processor(s) x and y that pertain to the signals x and y. 
     Selection 3) The unit is set to sampler mode, as for Selection 2 above, and then becomes the two channel device  12  of FIG. 7, fully described above. 
     FIGS. 9 and 10 show an embodiment that includes the use of a modified, pre-existing effects processor (not shown). A device used for this example may be an OEM reverb/effects unit from ART (Applied Research and Technology, Inc.), with two separate input/output channels, marked L and R in  108  and  104 . (A mono mode is available if longer loop times are needed with the given size of the on-board memory, though this reduces functionality to a single signal). The ART unit has 255 preset functions arranged into 16 banks selected with data wheel  101 . One bank is used for sampling, with 8 pairs of presets, each pair comprising a recording function and a playback function, with its own loop time corresponding to a metronome marking. A loop time T is selected from the preset pairs via data wheel  102 . Sampling is enabled by creating a continuously recording function, using the unit&#39;s one-tap delay at length of T-50 ms (milliseconds), with no feedback (=0% regeneration). This makes the output signal equal to the input signal, but it comes out T-50 ms later. Trigger circuit  92  monitors the loop&#39;s output so that mode switching occurs after the full waveform has been written to memory. When the trigger device  92  is set by reset switch  105 , it waits for this output to go above a preset threshold, illustrated by the sine wave  90  crossing a dotted line. The trigger output is then active until reset by reset switch  105 . A logic gate  93  uses the trigger output signal as the least significant bit in the preset pairs for sampling. An active trigger signal switches the unit&#39;s preset from sample record to playback by toggling the least significant bit. Gate  93  uses the least significant bit from preset data wheel  102  for all other (non-sampling) banks. Playback-type presets change from recording-type presets by turning off the input, changing regeneration to 100%, and setting the loop time to T. This makes the output signal equal to what the output signal was T ms before, effecting a continuously played loop of length T. The method of triggering from the output combined with the play loop&#39;s extra 50 ms time adds the 50 ms to the beginning of the record loop, so that the beginning of the signal is not cut off by the triggering mechanism (the entire triggering sequence takes less than 50 ms to accomplish). 50 ms is chosen for this example because it is the length of time required to capture a 20 Hz signal, the longest waveform in practical audio use. The trigger circuit also generates a control flag  109  during the wait state of recording, to be used for multi-channel switching schemes. During the wait state the trigger circuit flashes an LED at a tempo determined by independent metronome  106  via rotary switch  107 . In playback it is constantly lit; otherwise it is off. Also the system includes a pre-emphasis  108  and post-de-emphasis  104  of mid to high frequencies, to ensure a smooth splice from the end of one loop to the beginning of the next by effectively filtering out unnatural clicks from sudden voltage changes. 
     FIGS. 11-14 show some details for accomplishing the method of FIG.  7 . While a two-channel preview-sampler can be used as a stand alone device by cabling inputs and outputs to insert it appropriately within the signal path, the sampling and tuning process is significantly easier to use when integrated with a multi-channel signal mixer and automatic signal switching system, as shown in FIG.  7 . Two versions are discussed here. FIGS. 11 and 12 show a design that is to be built into a signal mixer (e.g., as part of the manufacturing process). FIG. 13 shows a stand-alone unit, to include an automatic switching system and signal interface, designed to be externally connected to a typical pre-existing signal mixer. FIG. 14 shows one implementation of a selection control device, for use with either of the two versions. 
     For this purpose, a switching network should accomplish the following tasks: 
     1. input the pre-processed signal into sampler channel A, 
     2. output the sampled signal of channel A into the processor to be tuned, 
     3. input the final post-processor mix of other signals, minus the tuning signal, into sampler channel B, and 
     4. output the sampled mix of channel B. 
     The output levels of channels A and B must be the same as in the final mix (the same as if there were no sampling system in place at all). For the dual channel (x &amp; y) scenario of FIG.  8 &#39;s Selection 2, the switching network would only need to perform tasks 1 and 2 noted above for both sampler channels A and B. The accommodation of all three selections of FIG. 8 (normal effect processing, two signal sampling, and single signal plus mix signal sampling) makes for a complicated array of switching. 
     FIG.  11  and FIG. 12 a  show a block diagram and detailed schematic of an implementation of the automatic switching process. One circuit block  110  is needed for each signal channel. To implement these switching modes requires at least a Double Pole Double Throw (DPDT) on-off-on switch  111  and a Single Pole Double Throw (SPDT) on-off-on switch  115 , as well as a DPDT on-off-on switch  117  at the output of the preview sampler circuit  12 . All switches can be digitally controlled. FIG. 12 b  shows the pin-outs for the 4053 switches of FIG. 12 a . The switches respond to four one-bit control signals: two universal mode messages, “Sample” (SMP) and “Single/Dual,” (2CH) and two channel specific messages, “Channel Selected” (CS) and “A/B Path” (A/B). The table summarizes the functions described below. 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Mode 
                 SMP 
                 2CH 
                 CS 
                 A/B 
                 SW 111 
                 SW 115 
                 SW 117 
               
               
                   
               
             
             
               
                 Normal 
                 L 
                 L 
                 L 
                 L 
                 OFF 
                 UP: Reverb In 
                 UP: Reverb Out 
               
               
                 Single + Mix Channel 
                 H 
                 L 
                 L 
                 L 
                 OFF 
                 DN: Mix B In 
                 DN: Mix B Out 
               
               
                 Channel Select (A) 
                 H 
                 L 
                 H 
                 L 
                 DN: A I/O 
                 OFF 
                 DN: Mix B Out 
               
               
                 Dual Channel 
                 H 
                 H 
                 L 
                 H 
                 OFF 
                 OFF 
                 OFF 
               
               
                 Channel Select,A 
                 H 
                 H 
                 H 
                 L 
                 DN: A I/O 
                 OFF 
                 OFF 
               
               
                 Channel Select,B 
                 H 
                 H 
                 H 
                 H 
                 UP: B I/O 
                 OFF 
                 OFF 
               
               
                   
               
               
                 H = logic level high, L = logic level low, UP = up/DN = down position in diagram, OFF = not conducting  
               
             
          
         
       
     
     Switch  111  of a particular signal mixer channel  110  is used to direct signal into and out of the preview sampler circuit  12  when that channel is selected for tuning. It responds to “Channel Select,” which activates it, and “A/B Path,” which chooses the sampler channel to be used. Otherwise it is off. 
     In “Single+Mix” mode, the selected signal&#39;s Switch  111  is set ON to Channel A sending signal from input  10   n  (shown here as a trim circuit) to the mix amp  116 . The output signal of preview sampler  12  is routed to amp  124  and subsequently to the signal processor  13 . It is set to OFF at all other channels. 
     In Dual Channel mode, two selected signals have their Switch  111  set to ON, one for Channel A and another for Channel B, allowing the tuning of a signal processor  13  for two different channels simultaneously. It is set to OFF at all other channels. 
     In “Normal” mode, every Switch  111  is OFF (the sampler is not used at all). 
     Switch  115  sends a post-processor and post-fader signal  112  to a mix at the sampler&#39;s Channel B input  116 , before or after an additional fader  113 . OFF sends no signal. 
     In “Single+Mix” mode, Switch  115  is ON for all signal mixer signals but one; it is OFF for the signal selected for tuning in Channel A. Thus, Channel A has the signal being tuned, and Channel B has a mix of everything else. Previewing with Channel B off and on provides a comparison of how the mix affects the tuned signal. 
     In “Dual-Channel” mode, every Switch  115  is OFF since no mix is needed. 
     In “Normal” mode, every Switch  115  can be set to send a post auxiliary fader signal to an auxiliary bus  114 , to allow the device used as preview sampler  12  to be used for another purpose, e.g. for reverberation. Note that in the example of FIGS. 11 and 12, we assume the effect unit to be set to synthesize a stereo reverb field from a monophonic source signal. This is a common technique, and allows for a simplification of the drawings. Thus, in “Normal” mode, Auxiliary bus  114  is a single channel that feeds only the B input of Preview unit  12 . 
     Switch  117  interfaces the sampler outputs with the Left/Right master output bus  118 . 
     In “Single+Mix” mode, the mono mix of Channel B is sent to both Left and Right outputs at unity gain, and Channel A does not directly reach the master bus at all. Channel A is inserted into the selected signal&#39;s mixer path by its Switch  111 , and reaches the output in that manner. 
     In Dual-Channel Mode, both Channels A and B function as inserts to two signal mixer paths, and no mix is involved, so no direct connection to the master bus is made. 
     In “Normal” mode, the output signals of device  12  can be sent to the master bus (A to Left and B to Right) via a pair of faders. 
     FIG. 13 is a schematic of the second version mentioned above, which is a stand-alone unit which includes an automatic switching system and signal interface, and is designed to be externally connected to a typical pre-existing signal mixer and signal processors. This function is similar to that of FIG. 12, requiring the same control data as well as a control signal  128  from the sampler to switch  111  with an added function that mutes the selected channel in record mode. When recording, there is no output signal at amp  125 , so no direct signal can appear in a mix somewhere down the line. This feature is useful in “Single+Mix” mode (described above) where a total mix without the selected channel is desired. However, this mix is not automatically created in this version and must be supplied by the user instead. The inputs for the mix are amps  126 . Here switch  127  mainly directs inputs, creating a mono mix from inputs  126 A and  126 B for “Single+Mix” mode for channel B of preview sampler  12 , as well as directing the output of preview sampler  12  channel B to both main outputs  123 A and  123 B. An additional resistor network (not shown) can reduce the input gain of the mono mix to prevent possible overload when two similar signals at inputs  126 A and  126 B are combined. In dual channel mode, the inputs  126  are muted, and the main outputs  123  are immaterial. 
     A type 4053 IC Triple 2-Channel Multiplexer with Inhibit can be used for each switch  111 ,  115 ,  117 , and  127  in FIGS. 11,  12 , and  13 . The pinout diagram is given at FIG. 12 b . Note that there are two physical switches for the single switch function labeled Switch  115 . 
     FIG. 14 is a schematic diagram of one possible implementation of a logic device to be used with the examples of FIGS. 11,  12 , and  13 . The schematic of control data generation demonstrates a centralized user interface. The user selects the desired channel from an input device such as a keypad, data wheel, or scroll switch. (Another arrangement would provide channel selection at each channel placement on the signal mixer, similar to a solo switch). With added logic, bi-directional 8-bit 74F299 shift registers  141  and  142  are arranged to provide 1-of-9 data selection for directing signals in and out of sampler  12  channels A and B (not shown). Register  141  either allows normal operation (SMP Low) or sets channel A to an individual channel signal. Register  142  sets channel B to an individual channel signal (2CH High) or a mix when register  141  indicates sample mode (SMP High). These registers are manipulated with a four-button keypad  143 , also labeled S 1 , S 2 , S 3 , S 4 . The register direction—scroll up or down—is set with SR latch  144 , and the retriggerable monostable multivibrators  145  and  146  provide a clean clock pulse to registers  141  and  142  respectively for sequential scrolling. The channel and mode selection of registers  141  and  142  are converted by logic gates  147  into control data CS′ and A′/B 1˜8 for the signal switches described above. Channel selection can be displayed with LED bargraph  148 . 
     Use of the present invention may have a particular advantage for detecting an intermittent fault in a device (e.g., a broken electronic device). Many devices develop faults that generate a spurious noise intermittently, sometimes only once every few hours. Waiting for this to happen is a problem for a technician, who usually will need to perceive the problematic behavior in order to determine its cause and solution. Using the preview sampler with threshold detection can record such an occurrence without the presence of a technician, who can analyze the recorded data after the occurrence. Additionally, a ‘ready’ signal, such as a light or sound, may be used to alert the user that the event has occurred. The threshold level is set above the ‘noise floor’ of the device with a null signal as input, and will thus trigger only when the spurious noise appears. 
     Where a test signal is desired or required for this use, the threshold input of the preview sampler may be fitted with a mixer that sums the input received from the device being tested to an inverted test signal directly from the signal generator. By balancing the relative levels in the mixer, the two can be summed to zero, and the threshold level set to the noise floor, as above. The trigger will go off only when the device being tested changes its behavior. 
     A second scenario is the use of a multi-channel embodiment, such as those of FIGS. 3,  4 , or  5 , to simultaneously record different parts of a circuit. This allows the technician to compare the circuit&#39;s behavior at different points and to determine which sections are behaving improperly. A switch at the preview sampler&#39;s output would facilitate viewing each recorded sample on an oscilloscope, for example. This scenario is particularly helpful when combined with the detection/correction of an intermittent error discussed above.