Verticle TV & Music

CHOICES FOR DIGITAL AUDIO

If you're going to install AES digital audio, you have a number of choices.  The first choice is to decide how to ship this audio around your facility, embedded or separate audio and video?. 

Embedded audio is simple.  One coax cable will carry the video and also be carrying the audio.  This simplifies the labor by half, since you're only using one connector (a BNC) to carry both signals. 

The second advantage is lip sync.  If the audio and video were in sync when they were put on the cable, they will stay in sync until they come off the cable.  And loudness is also a non-issue: if the loudness level of the audio was perfect when put on the cable with the video, it will stay at that level until you take the signal off.

In fact, the only real disadvantage to embedded audio is that it is embedded.  You can't do anything to the audio without "de-embedding" it first.  This requires an expensive box.  But, if you need to adjust the audio, lip sync, volume or content, they you will have to de-embed the audio to work on it.  Of course, once it is de-embedded, then the audio runs on a separate cable. And then you have more choices…

In fact you have five choices (or possibly even more) at that point.  Table 1 lists all the basic choices you have.

CABLE TYPE                             CONSISTS OF                                   STANDARD

Analog audio twisted pairs         standard analog audio cable.             None

Digital audio twisted pairs          110 Ω, low capacitance digital cable   AES3

Category cable                          100 Ω Category 5e, Category 6 etc.   None for audio

Coax cable                                 75 Ω video coax                                  AES3-id

Ethernet                                     4-pair                                                  EIA/TIA 568B.2

Table 1

Let's look at each of  the choice in Table 1.

ANALOG AUDIO TWSITED PAIRS

Why can't you use your old cable to carry this signal?  Let me show you what happens if you run digital audio on analog pairs.  Figure 1 show the effect at 15 metres.

Figure 1

The perfect square is the original digital bit stream, in this case a 48 kHz sampled bit stream (bandwidth 6.144 MHz). The dotted line is the end of 15 metres of digital cable (Belden 1696A). The other trace is an analog cable (Belden 8451).  Obviously, these two signal are not the same but they are both probably recoverable.

But, when you get to 30 metres, in Figure 2, things don't look so good.

Figure 2

You can see that the signal voltage, more than +/- 2 volts in the original bit stream, is now down to half that (+/- 1 volt) on the analog cable.  But even worse than that is the rise and fall time on the analog cable. 

The reason why this is critical is because that rise and fall is the AES clock.  The clock is  extracted from the signal and used to decode the signal itself. The problem is that, if you look closely at the rise time of the waveform on the analog cable, where is the transition between zero and one?  It is very clear on the original waveform, and pretty good, on the digital cable.  But on the analog cable, the receiving chip might have a hard time using this as the clock.  This is called ‘jitter', anything that affects clock timing.

Analog cable is not designed for digital audio.  It's not the right impedance (110 Ω).  It's more like 30 to 40 Ω.  This is an impedance mismatch.  Such a mismatch causes reflection ("return loss") on the cable.  As Figure 2 shows, half of the signal is reflected back to the source and lost at the destination (load).

The analog cable is also not right capacitance.  Digital cable is 43 pF/m.  Analog cable is more than 100 pF/m.  That capacitance is why we have than long charge-up and discharge, confusing the clock recovery and adding jitter. So the conclusion of this is, don't use analog cable to run digital.  If you don't have any choice, keep cables short, less than 15 metres.

DIGITAL AUDIO ON COAX

AES3-id is the standard for running digital audio on coax.  TV broadcasters insisted on this option because they could use the same cable to run audio and video.  One cable, one connector, one strip tool, one crimp tool.  Sounds pretty easy.  And BNC's are a lot easier to install than XLR's. 

These days, broadcasters often put AES3-id audio on miniature coaxes.  These can often go farther than twisted pairs.  In fact, if you want to have long runs of digital audio, put it on coax.  It can double or even triple the distance achievable on twisted pairs.

CATEGORY CABLES

Running digital audio on Category 5e or 6 is quite common these days.  I suppose it's because designers and installers realize this cable is intended to carry data (Ethernet) so using to carry digital audio is not such a big jump.

The only problem is that Category 5e and 6 cables come four pairs to a cable.  If you want four channels of digital audio, I suppose this would be fine. (And, in digital audio, you can also run two channels per pairs, so this could also be eight channels of digital.) But what if you only want one channel, or at least one at a time?  This is the reason for Belden 1353A, one-pair Category 5e, specifically made for non-data applications with a single bonded pair 24 AWG, unshielded.

ETHERNET

This last choice is somewhat unlikely, but there are a lot of audio equipment manufacturers who are now offering Ethernet as a way to transport audio.  They can run 32 channels or more (depending on the sampling rate), and you can use standard Ethernet® switches and other support devices, even though the payload is audio.

Then, of course, you cable is Category 5e or Category 6 doing what it was designed to do, carry Ethernet. So it's interesting that Category 5e or 6 could carry AES digital audio in its "native" format, and also when it runs as Ethernet.  Lots of possibilities!

About the Author

Steve Lampen has been with Belden for 19 years and currently is Multimedia Technology Manager and Product Line Manager for Entertainment Products.  He has an FCC Lifetime General License (formerly an FCC First Phone License) and is a BICSI Registered Communications Distribution Designer. His latest book "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill.

 For more info visit our web site www.beldenapac.com or email us info@beldenapac.com

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