Receiver Interface blues

[bug_new]

I have been out of the audio/video analysis game for a decade or so, and have recently been drawn in again finally by the desire to bump up to digital recording and playback. I want to put a digital media center together (DVR) with a 7.1ch receiver and a digital HDTV.

The first stumbling block I discovered, was the cable interfaces. There were still the old analog audio (L+R) and the three wire XLR, and a pile of analog video options: component, composite, s-video, vga, and a couple of others, as you might expect for legacy gear, and a myriad of new digital interfaces [SPDIF, TOSLINK, DVI and HDMI, and firewire]. I was at first impressed at the newer interfaces, abut later dismayed by them.

Unless I am mistaken, the new digital interfaces are all flawed.

The first seems to have been the audio optical TOSLINK interface, that rather than using laser and fiber technologies to gain distance, immunity and bandwidth, uses LEDs and plastic, and can only function out to 5m or so. The copper equivalent seems to have followed and is a single ended connector, with RCA (not and RF 50ohm or 75ohm connector), and hence it is also limited to like 5m and is subject to noise and ground loops.

Digital video seems to have taken longer to arrive, starting with DVI. This seemed like a good interface, since it housed both analog and digital pins in the same connector, making the transition (especially in the computer world) easy. It is very large, making it very difficult to run through conduit, and is again single ended signaling, so that it is limited to 5-10m. DVI also does not contain any audio signals., so a separate cable run is needed for this.

IEEE 1394 (firewire) seems to be bandwidth limited, so it may have limited use, even at 800Mbps.

The next connector on the scene seems to be HDMI. This connector was an all digital interface (dropping the analog video), smaller connector, that includes the digital audio line. The big issue here, is that it is still a single ended interface, again limiting it’s run length to less than 20m, allowing for ground loops and common mode interference injected on all of the wires. As far as I can tell, no one makes a cable that forms HDMI from a DVI and SPDIF connector. Why not, isn’t this how this formed? They seem to make DVI to HDMA converters, but what happens to the audio signal (and the analog video signal for that matter)? Doesn’t it make sense to combine the larger DVI + dig audio into the smaller HDMI cable run to the HDTV or receiver?

Why are these all single ended versus differential cables?

If these bugs were worked out, wouldn’t I just need a one or two input to one output (HDMI to HDMI) receiver and an HDMI HDTV? If long runs are needed, it seems to me that nothing is in place (in the digital world) for easy long runs, since none of these are differential signaling (unlike the XLR). Adding the ability to have up-conversion in the receiver, allows for legacy analog gear to be attached with ease, though I would think that since everything is within the DVR (several tuners, DVD, CD, music library, etc), what else is there to connect?

Is there a digital receiver out there, that takes a couple of HDMI inputs, and outputs one HDMI output, with 7.1 digital audio with say 150W/channel?

-Bug_new

 

[MDS]

Unless I am mistaken, the new digital interfaces are all flawed.

You are mistaken.

The first seems to have been the audio optical TOSLINK interface, that rather than using laser and fiber technologies to gain distance, immunity and bandwidth, uses LEDs and plastic, and can only function out to 5m or so.

You can buy glass optical cables instead of plastic if you really want to spend the extra money (for zero gain). There is no need to use anything other than a relatively low power emitter because the distance in a typical home theater is not great.

The copper equivalent seems to have followed and is a single ended connector, with RCA (not and RF 50ohm or 75ohm connector), and hence it is also limited to like 5m and is subject to noise and ground loops.

The s/pdif specs require a 75 Ohm characteristic impedance and any 75 ohm cable will work just fine - that is any cable designed for composite or componet video or specifically labeled 'digital coax'.

Digital video seems to have taken longer to arrive, starting with DVI. This seemed like a good interface, since it housed both analog and digital pins in the same connector, making the transition (especially in the computer world) easy. It is very large, making it very difficult to run through conduit, and is again single ended signaling, so that it is limited to 5-10m. DVI also does not contain any audio signals., so a separate cable run is needed for this.

There are three different types of DVI: DVI-A, DVI-D, and DVI-I. Only DVI-I can be used for either analog or digital. A and D are analog and digital only, respectively. DVI was designed for video and adopted from the computer industry. We can't now say that it was a poor choice not to include audio because it was designed for video.

IEEE 1394 (firewire) seems to be bandwidth limited, so it may have limited use, even at 800Mbps.

Not sure what you are getting at here. An 800 mbps data rate far exceeds any current requirements.

As far as I can tell, no one makes a cable that forms HDMI from a DVI and SPDIF connector. Why not, isn’t this how this formed? They seem to make DVI to HDMA converters, but what happens to the audio signal (and the analog video signal for that matter)? Doesn’t it make sense to combine the larger DVI + dig audio into the smaller HDMI cable run to the HDTV or receiver?

Simply because that doesn't make sense. S/PDIF is the protocol for transmitting digital audio. In the case of s/pdif over coax, the s/pdif spec also defines the electrical characteristics of the physical interface. In the case of optical connections, the protocol is still s/pdif, but now the physical interface is known as toslink (which is also the name of the fragile little plastic connector).

If you were to use a dvi to hdmi converter, you will not get audio because the audio doesn't exist as the dvi interface is for video. HDMI defines the protocol and physical interface for carrying both audio and video data. DVI + s/pdif accomplishes the same thing but would be two cables and two different protocols and physical interfaces. HDMI is meant to simplify that with one cable.

Yes there are new models of receivers that include multiple hdmi inputs and outputs.

 

[bug_new]

Simply because that doesn't make sense. S/PDIF is the protocol for transmitting digital audio. In the case of s/pdif over coax, the s/pdif spec also defines the electrical characteristics of the physical interface. In the case of optical connections, the protocol is still s/pdif, but now the physical interface is known as toslink (which is also the name of the fragile little plastic connector).

If you were to use a dvi to hdmi converter, you will not get audio because the audio doesn't exist as the dvi interface is for video. HDMI defines the protocol and physical interface for carrying both audio and video data. DVI + s/pdif accomplishes the same thing but would be two cables and two different protocols and physical interfaces. HDMI is meant to simplify that with one cable.

I am not sure that I follow your reasoning here, but I will take a shot at it. I think that you are saying that the HDMI interface is a protocol as well as an interface, as is the copper S/PDIF protocol and interface, and these two protocols are incompatible. If this is the case, there can be several reasons, such as not having a separate audio pin within the HDMI interface, but rather encapsulate the audio within the frame somewhere. It could also still have a separate pin, but not have a compatible format. DVI must have a compatible format, since I suspect that the DVI to HDMI converters are passive devices.

It would be nice to see an economical active device that accepts coax s/pdif and DVI, and creates HDMI, creating a smaller easier to run line to the remote display (with sound), or for the digital video/audio output run from the noisy PC in the other room to the receiver (50 to 100 feet away by the time the cable is run).

 

[MDS]

It would be nice to see an economical active device that accepts coax s/pdif and DVI, and creates HDMI, creating a smaller easier to run line to the remote display (with sound), or for the digital video/audio output run from the noisy PC in the other room to the receiver (50 to 100 feet away by the time the cable is run).

A receiver that transcodes all inputs to HDMI is such a device. That is for backwards compatability with devices that don't have HDMI outputs. The goal of HDMI is to have one cable for both audio and video.