Abstract:
Digital delay apparatus for variable delay uses a shift register fixed delay driving a random access memory variable delay. A variable modulus counter controls the read-write addressing, whereby the difference between addresses sets the variable delay of the random access memory. The random access memory capacity is small relative to the shift register for increased efficiency.

Description:
The present invention relates to digital circuits, and more particularly to delay circuits. 
     When processing digital signals, it is known to pass the signals through a shift register with the size of the shift register determining the delay. Such an arrangement is included in U.S. Pat. No. 4,051,531. By `size` is meant the number of stages through which digital signals pass when going through the register. The difficulty with this arrangement is that manufacturers tend to make shift registers only in set sizes which normally differ in size by a factor of 2. Thus, if a delay of say 200 bits is required it would be necessary to use a combination of shift registers of differing sizes in order to provide such a delay, for example one 128-bit shift register, one 64-bit shift register and one 8-bit shift register. This can be expensive and also consume large amounts of space. 
     SUMMARY OF THE INVENTION 
     The present invention provides a random access memory device to provide at least part of a desired delay. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Features and advantages of the present invention will be more apparent from the following description of embodiments thereof, given by way of example when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a block diagram of the basic apparatus used in one embodiment of the present invention; 
     FIG. 2 shows a more detailed block diagram of the embodiment shown in FIG. 1; 
     FIG. 3 shows diagrams which assist understanding of the block diagram of FIG. 2; 
     FIG. 4 shows diagrams which assist understanding of another embodiment of the present invention; and 
     FIG. 5 shows diagrams which assist understanding of yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the preferred embodiment of the present invention is based on a combination of a standard shift register 10 and a memory device 11. The register 10 provides part, and preferably the major part, of the desired delay with the remainder of the delay being provided by a random access memory device (RAM) 11. This arrangement provides a long delay using the minimum of apparatus. If only a short delay which is not a power of 2 is required, the RAM could be used alone. 
     FIG. 2 shows in block diagram form how the arrangement shown in FIG. 1 can be put into practice. A typical example of the use of the apparatus shown in FIG. 2 is when digitally recording a PAL television video signal by sampling the video signal at a multiple, e.g. twice subcarrier frequency (2f sc ). FIG. 2 will therefore be described in relation to such a use. 
     When digitally recording a PAL video signal by sampling at 2f sc  it is necessary to provide a one line delay, but there are 283.7516 cycles of PAL subcarrier per line. Hence for video sampled at 2F sc  there will be 2×283.7516=567.5032 samples per line. A one line delay would therefore be required for 567 or 568 bits. Using the apparatus shown in FIG. 2, this can be obtained by combining a commercially available 512 bit shift register 10 with the RAM 11, the RAM providing a delay of 55 or 56 bits. 
     The RAM 11 is a standard read/write RAM of a capacity of at least the desired delay, e.g. 56 bits the RAM being addressed by a variable counter 12 of modulus equal to the required delay for the RAM. The modulus of the counter 12 is set by applying a signal indicative of the required delay, e.g. 55 or 56, on the data input terminals of the counter. The counter 12 is shown as comprising two sections each capable of producing a 4-bit output. For a delay of 55 or 56 this is necessary but of course more of less sections may be required for different delays. 
     FIG. 3 shows the timing diagram for the RAM 11 giving n clock pulses delay by addressing the RAM using a modulo-n counter. During the time occupied by one clock period, the RAM is first put into the `read` mode, and then the `write` mode. Hence, information which was written n clock pulses before is obtained, and the new information is written, ready to be accessed n clock pulses later, giving the required delay. 
     For some applications, otherwise suitable shift registers and/or random access memories may be incapable of operating at the required speed. In this case m parallel paths operating at 1/m of the data rate may be used to overcome this problem. Using any one of the above embodiments, delay periods which were previously difficult or expensive to provide can now be easily obtained. 
     The additional delay can be accomplished in a number of ways depending on the type of RAM used and the way in which it is addressed. In FIG. 2, which shows diagrams which assist understanding of another form of apparatus which could be used as the apparatus shown in FIG. 1. A standard single port read/write RAM is used to provide the additional delay. Using this type of device, data is written into and read from the RAM in the space of one clock period. 
     The write and read addresses are arranged to be separated by an interval of n by suitable external arithmetic of the correct modulus. Referring to FIG. 4, it can be seen that data is written into the RAM during the first half of the clock period at, for example address x. During the second half of the clock, data is read from the memory at address x-n. This then retrieves data which was written n clock pulses ago, and therefore gives the required delay. The delay may be varied simply by changing the value of n in the arithmetic which generates the read and write addresses. 
     An alternative RAM which could be used is a two port RAM and FIG. 5 shows the relationship between read and write addresses to give n clock pulses of delay in a two port RAM. A two port RAM has the useful property that the memory cells are designed so that data may be written into and read from the memory at the same time. Two sets of addresses are required; a read address and a write address, and these addresses both increment on each clock pulse. By using a suitable counter system or arithmetic, it can be arranged that the read and write addresses are always a certain numerical value n apart. The difference between read and write addresses then gives the number of clock pulses delay through the RAM.