Abstract:
An HDMI interconnect arrangement is presented that performs a pulse-amplitude modulation (PAM) conversion of the TMDS audio/video signals in order to simultaneously transmit all three channels over a single optical fiber. The set of three audio/video TMDS channels is applied as an input to a PAM-8 optical modulator, which functions to encode the set of three channels onto an optically-modulated output signal. The modulated optical signal is thereafter coupled into an optical fiber within an active HDMI cable and transmitted to an HDMI receiver (sink). The TMDS CLK signal is not included in this conversion into the optical domain, but remains as a separate electrical signal to be transmitted along a copper signal path within the active HDMI cable.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/186,821, filed Jun. 13, 2009 and herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an HDMI interconnect arrangement and, more particularly, to an optical fiber-based HDMI interconnect for use with TMDS signals that performs a PAM-8 conversion of the TMDS data channel signals in order to simultaneously transmit all three channels over a single optical fiber. 
     BACKGROUND OF THE INVENTION 
     HDMI is a specification that integrates audio and video information into a single digital interface to be used with, for example, digital video disc (DVD) players, digital television (DTV), high definition TV (HDTV), set-top boxes, and other audio and/or video devices. An HDMI interface cable is configured to carry multi-channel audio data, as well as standard and high definition consumer electronics video formats. Content protection technology is also available and an HDMI cable may be configured to carry control and status information in both directions. 
     Transition minimized differential signaling (TMDS) is a signaling technique used in HDMI to produce a transition-controlled, DC-balanced series of symbols from the audio and video inputs. TMDS is more properly a way of encoding the audio and video digital information to protect it from degrading as it travels along the length of the HDMI cable, creating data channels associated with the three original colors (RGB)/intensity aberration (YPbPr), defined as TMDS channels  0 ,  1  and  2 , as well as a video pixel clock, defined as TMDS CLK. In a long string of logic zeros and logic ones, bits are selectively manipulated in TMDS in order to keep the DC voltage level of the overall signal centered around a particular signal threshold. Another signal threshold at the receiver is then used to determine whether a received data bit is at a voltage level equivalent to a logic zero ( 0 ) or at a level equivalent to a logic one ( 1 ). 
       FIG. 1  illustrates a conventional prior art implementation of an HDMI system, showing an HDMI source  1  and an HDMI sink  2 , coupled together via an HDMI cable  4 . Within HDMI source  1 , the associated video and audio input signals are applied to an HDMI transmitter  3 , which transforms these signals into a set of three signals along TMDS channels  0 ,  1  and  2 , and a separate TMDS CLK signal. The TMDS CLK signal will be used by HDMI sink  2  as a frequency reference for data recovery on the three audio/video channels. In the particular arrangement shown in  FIG. 1 , this set of four signals are transmitted as simplex signals (i.e., “one-way” transmissions) along copper wires within a standard HDMI cable  4  and transmitted to HDMI sink  2 . Other copper wires within HDMI cable  4  are used to support half-duplex transmission (i.e., “two-way” transmission at different times for each direction) of various other signals used in HDMI signaling (DDC, CEC, VDD, GND, etc.). The simplex transmission wires supporting the TMDS signals are thereafter coupled, as shown in  FIG. 1 , into an HDMI receiver  5  that functions to re-convert the TMDS signals into the proper video and audio signals (as well as the recovered clock signal). 
     In many conventional transmission/reception systems using HDMI, cable  4  comprises a purely electrical cable (usually composed of copper wires). However, there are major drawbacks associated with the use of an electrical cable as a result of its susceptibility to electromagnetic interference, mainly due to the limited bandwidth of the copper wire. Furthermore, the signals transmitted along copper wire are susceptible to power loss, thus limiting the length of copper-based electrical HDMI cables to less than about 15 meters. 
     It has been previously suggested to replace copper-based HDMI cables with optical fiber-based HDMI cables. By virtue of using an optical cable, the data rate and cable span may both be significantly increased while avoiding the problems of copper wire. Specific arrangements of these optical fiber-based alternatives are disclosed in U.S. Published Application No. 2007/0233906, published on Oct. 4, 2007. To date, however, these solutions pose their own limitations in supporting the transmission of the multiple TMDS channels. One solution is to use separate fibers for each channel, another solution is to use separate wavelengths for each channel. Either of these solutions introduces additional components and/or sources of loss into the system, increasing its complexity and cost. Another proposed solution is to “serialize” the set of TMDS channels prior to transmission over a single fiber, and then “deserialize” the data at a receiver unit (a technique referred to in the art as SERDES). The proposed SERDES solution, however, creates other problems in terms of requiring the HMDI connection to operate at three times the data rate of the individual channels (for example, at 10.2 Gbps as compared to 3.4 Gbps) and as a result requires the use of expensive, high bandwidth components that may consume more power than their lower speed counterparts. 
     Thus, a need remains for a optical fiber-based HDMI cable that addresses the concerns of the multiple fiber/multiple wavelength arrangements without requiring the use of more complex components associated with SERDES operations. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention, which relates to an HDMI interconnect arrangement and, more particularly, to an optical fiber-based HDMI interconnect for use with TMDS signals that performs a pulse-amplitude modulation (PAM) conversion of the TMDS audio/video signals in order to simultaneously transmit all three channels over a single optical fiber. 
     In accordance with the present invention, the set of three audio/video TMDS channels is applied as an input to a PAM-8 optical modulator, which functions to encode the set of three channels onto an optically-modulated output signal. The modulated optical signal is thereafter coupled into an optical fiber within an active HDMI cable (where an “active” HDMI cable is defined as a cable including both optical fibers and copper wires and necessary optical-to-electrical (O/E) conversion components), and is transmitted to an HDMI receiver (sink). The TMDS CLK signal is not included in this conversion into the optical domain, but remains as a separate electrical signal to be transmitted along a copper signal path within the active HDMI cable. The PAM-8 signal propagating along the optical fiber is then recovered at an HDMI sink and decoded (using the separately-transmitted TMDS CLK signal) to reconstruct the set of three TMDS channels. Known HDMI receiver techniques are then used to translate the channels into the desired video and audio information. 
     The utilization of PAM-8 modulation to transmit TMDS channels in accordance with the present invention eliminates the need for extra fibers or extra wavelengths (as used in the prior art), while also maintaining the original data rate of the TMDS data and thus avoiding the need for high-speed electronics (as used in the SERDES-based prior art arrangement). Advantageously, the active HDMI cable formed in accordance with the present invention may be used as a direct replacement for conventional electrical HDMI cables (or as a replacement for the various types of active HDMI cables described above). 
     In one embodiment, the active HDMI cable connection of the present invention utilizes multimode fiber, which is relatively inexpensive while still providing sufficient bandwidth to extend the distance over which HDMI signals may travel when compared to the conventional copper HDMI cable. In an alternative embodiment, single mode fiber may be used and is considered to be preferred when transmitting the signals over distances in excess of 100 meters (compare to the 15 meter limitation of HDMI copper cable). The fiber itself may comprise either a suitable glass or plastic material. 
     In accordance with one embodiment of the present invention, a Mach-Zehnder interferometer (MZI) is utilized as an optical modulator to directly create the PAM signal in the optical domain. In this case, the original TMDS data channels are first phase aligned to form clocked data streams, with the clocked data then applied as the electrical input signals to the optical modulator. Indeed, the clocked data may be further encoded to create an N-level input signal (N&gt;3) and a multi-segment modulator utilized to improve the linearity of the response over the range of phase modulation. 
     Alternatively, a vertical cavity surface-emitting laser (VCSEL) may be directly modulated with the phase-aligned data signals to create the PAM-8 optical output signal. 
     In accordance with another embodiment of the present invention, the TMDS-to-PAM conversion may be performed in the electrical domain, with the PAM-8 electrical signal thereafter converted into an optical signal. 
     Other and further embodiments and advantages of the present invention will become apparent during the course of following discussion and by reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Referring now to the drawings, 
         FIG. 1  is a block diagram of a prior art implementation of an HDMI source and sink, with a conventional HDMI cable connecting the source and sink, the diagram showing in particular the utilization of TMDS coding to transmit the video and audio signals; 
         FIG. 2  illustrates an exemplary PAM-encoded optical fiber-based arrangement utilized to transmit HDMI TMDS signals along an active HDMI cable in accordance with the present invention; and 
         FIG. 3  illustrates an exemplary optical modulator configured to convert the 3-channel TMDS signals into an optical PAM-8 signal for transmission over a single optical fiber in the arrangement of  FIG. 2 ; 
         FIG. 4  shows an additional feature that may be utilized to increase the linearity of the output response by encoding the original three TMDS signals into a plurality of N phase-aligned bit streams used as the input to the PAM optical modulator; and 
         FIG. 5  illustrates an alternative configuration for the active HDMI cable of the present invention where the three TMDS data channels are first converted into an electrical PAM-8 signal, with the electrical PAM signal then transformed into the optical domain prior to transmission from the HDMI source to the HDMI sink. 
     
    
    
     DETAILED DESCRIPTION 
     As mentioned above, there are various factors limiting the performance of HDMI cables, whether a pure electrical cable or a combination of optical fibers and copper wires (i.e., an “active” HDMI cable). The fiber-based connection of the present invention addresses these concerns by providing the ability to simultaneously transmit all three channels of the TMDS data signals over a single optical fiber without the use of multiple wavelengths or SERDES components by, instead, performing a pulse amplitude modulation (PAM) conversion on the TMDS data channels and transmitting the PAM signal over an optical fiber. 
     Reference is made to  FIG. 2 , which illustrates an active HMDI cable connection  10  that is proposed for use in place of HDMI cable  4  of the prior art. As will become apparent during the course of the following discussion, inventive HDMI cable connection  10  may be used as a direct replacement for existing HDMI cables (electrical cables or active cables) deployed in various systems. Referring to  FIG. 2 , active HDMI cable connection  10  is shown as including an optical fiber  12  to transmit channels  0 ,  1  and  2  of the TMDS data (these three channels are shown in a dotted box in  FIG. 1 ) as a PAM-8 optical signal. In particular, TMDS channels  0 ,  1  and  2  as created by HDMI transmitter  3  (see  FIG. 1 ) are first applied as an input to a phase alignment element  14  (which may comprise, for example, a D-type flip-flop circuit arrangement). The operation of phase alignment element  14  is controlled by TMDS CLK to create a set of aligned data signals, shown as bit- 0 , bit- 1  and bit- 2 , at the output of phase alignment element  14 . 
     The phase-aligned streams represented as bit- 0 , bit- 1  and bit- 2  are then applied as separate electrical inputs to an optical pulse amplitude modulator  16 . A continuous wave (CW) optical source  18  operating at a known wavelength λ is used to supply an optical input signal I to modulator  16 . As will be described below, modulator  16  functions to create a pulse amplitude modulated (PAM) optical output signal that is representative of the values of the original three electrical input signals—that is, representative of TMDS channels  0 ,  1  and  2 . In this case, a PAM-8 configuration is utilized, meaning that a total of 8 different amplitude levels are required to completely define all the various possibilities of the three digital input streams. 
     The output PAM-8 optical signal from modulator  16  is thereafter applied as an input to an optical fiber  12 , where optical fiber  12  is defined as one component of active HDMI cable connection  10 . The remaining signals, including TMDS CLK, ground, power, and the like, are transmitted in electric form over copper wires  20  also included in active HDMI cable connection  10 . The use of an active HDMI cable including both copper and fiber for connection  10  allows for the phase alignment, encoding and decoding at both the transmit and receive sides of the cable to be simplified, without sacrificing on the distance travelled by the signals, the signal integrity, or requiring additional/complicated circuitry to perform signal processing/filtering to removing timing jitter. Indeed, by using this technique of transmitting only the TMDS channels  0 ,  1  and  2  over an optical fiber, the electronics continue to operate at the 3.4 Gbps rate, as compared to the 10.2 Gbps rate required for the SERDES implementation mentioned above. 
     At the receive side, the PAM-8 signal propagating along optical fiber  12  is first re-converted into the electrical domain within an optical-to-electrical (O/E) conversion device  22  and then applied as an electrical input to a 3-bit A/D converter  24  to recover the digital data signals bit- 0 , bit- 1  and bit- 2 , as controlled by the received (electrical) TMDS CLK signal. Thereafter, the clocked data bits are applied as separate inputs to a TMDS output driver  26  to recover the original three TMDS data channels. 
       FIG. 3  illustrates an exemplary PAM-8 optical modulator  16  that may be used to convert the phase-aligned bit- 0 , bit- 1  and bit- 2  representations of the TMDS data channels into a phase amplitude modulated optical output signal. In this particular embodiment, modulator  16  comprises a multi-segment Mach-Zehnder interferometer (MZI)  30 . As shown, optical input signal I from source  18  is supplied to an incoming optical waveguide  32 , which then splits along a pair of waveguide arms  34  and  36 . In this case, a differential MZI is shown, with a plurality of segments formed along both waveguide arms  34  and  36 . That is, a first set of segments  38 - 1 ,  38 - 2  and  38 - 3  are formed along waveguide arm  34  and a second set of segments  40 - 1 ,  40 - 2  and  40 - 3  are formed along waveguide arm  36 . 
     Phase-aligned data streams defined as bit- 0 , bit- 1  and bit- 2  are shown as applied as the electrical inputs to segments  38 - 1 ,  38 - 2  and  38 - 3 , respectively, with their inverse values similarly applied as inputs to segments  40 - 1 ,  40 - 2  and  40 - 3 . The presence of these electrical signals will modify the properties of the optical signals propagating along arms  34  and  36 , introducing a fixed amount of phase delay between the signals propagating along each arm as a function of the logic “0” or logic “1” values of each bit at each point in time. These optical signals thereafter recombine along an output optical waveguide  42  to form the PAM-8 optical output signal O. A complete discussion on the use of an MZI to create such a PAM optical output signal can be found in U.S. Pat. No. 7,483,597 issued to K. Shastri et al. on Jan. 27, 2009, assigned to the assignee of this application and hereby incorporated by reference. 
     It is also possible to further encode the phase-aligned set of three bit streams to create a plurality of N separate data bit streams and utilized the N separate streams as input to a PAM modulator. By increasing the number of electrical inputs (and also, perhaps, increasing the number of segments forming an MZI modulator), the PAM output signal will be defined at an increased level of precision by improving the linearity of the transfer function of the phase response.  FIG. 4  illustrates this aspect of the present invention, where phase-aligned signals bit- 0 , bit- 1  and bit- 2  are first applied as inputs to an encoder  42  that is used to introduce an additional level of differentiation between the signals and generate an increased number of output (still phase aligned) data streams. In a simple arrangement, encoder  42  may generate a set of four output signals from the three input signals and an associated MZI would be formed to include an additional segment to receive the additional input. 
     While the arrangement of  FIG. 3  illustrates an embodiment of the present invention utilizing an MZI optical modulator, it is to be understood that there are other types of optical arrangements that may also be used to create the PAM-8 output signal. For example, as mentioned above, a directly-modulated VCSEL source may be used to generate the desired PAM-8 optical output signal. 
     As mentioned above, it is also possible to first create a PAM-8 signal in the electrical domain, directly from the phase-aligned bit- 0 , bit- 1  and bit- 2  signals.  FIG. 5  illustrates this aspect of the present invention, where a digital-to-analog converter (DAC)  50  is used to create a PAM-8 output signal from the parallel bit streams. This electrical PAM-8 signal is then applied as an input to an electrical-to-optical (E/O) conversion element  52 , which may simply comprise a lasing device. The output signal from E/O device  52  is therefore an optical version of the PAM-8 signal, which is thereafter coupled into optical fiber  12  in the same manner as described above. 
     When compared to implementations requiring the use of SERDES, the PAM-8 rate as used in the various embodiments of the present invention is much lower and enables the use of less expensive multimode fiber without any sacrifice in performance of the system. Indeed, the active HDMI cable of the present invention may be used as a plug-in replacement for any conventional HDMI cable already in use. Utilizing multimode fiber also has the extra benefit of providing greater tolerances for optical alignment, enabling lower cost, high volume manufacturing. The proposed arrangement of the present invention has the further advantage of extending the span over which the HDMI TMDS signals can travel to distances beyond 100 meters, depending upon the fiber type and various other operating parameters. Indeed, when using single mode fiber in place of multimode fiber, much greater distances can easily be achieved. The fiber itself will, in most cases, comprise a glass (silica-based) material. However, it is also possible to use certain polymer (plastic) materials in the formation of an optical fiber for this purpose. 
     While the present invention has been described with reference to different embodiments thereof, those skilled in the art will recognize that various changes may be made without departing from the spirit and scope of the claimed invention. Accordingly, the invention is not limited to what is shown in the drawings and described in the specification, but only as indicated in the claims appended hereto.