subject: DIGITAL AUDIO ON TWISTED PAIRS [print this page] I'm sure I don't have to tell you that the word is going digital. And, in the audio-video space, the first to go digital was audio. The Audio Engineering Society (AES) and the European Broadcast Union (EBU) worked on the first digital audio standards back in the 1980's, and the first standard, called AES3, came out in 1985, with continuing improvements until the present day.
Digital audio is a very different animal from analog audio. And, for the last twenty-five years, the forces of analog have been resisting the forces of digital audio. Now there will probably always be those who will use analog, just like there are still people who ride horses. A horse may not be as fast as a car, but it is a lot more maneuverable, its pollution is a lot more eco-friendly, and there are still people making saddles. In the same way, there will be analog microphones and mixers and tape machines made for a long time. The last of the large 24-channel multichannel professional machines, the Studer A827, was made in 2004. Smaller machines are still being made.
When digital first appeared there were many comments about the quality of the sound, especially compared to analog. But, as digital has improved over the last decade or so, the analog crowd has grown smaller and smaller. A lot of this is probably due to the MP3 and download crowd, who accept marginal quality as long as they get the content they want. Compared to that quality, true AES digital audio is significantly higher quality.
So what is AES3 digital audio? It is a system that allows the user to sample the analog material and turn it into digital audio (data). Table 1 shows the most common sampling rates.
SAMPLING RATE
WHERE IS IT USED?
ACTUAL BANDWIDTH
44.1 kHz
Consumer digital, S/PDIF
5.6448 MHz
48 kHz
Broadcast radio/television
6.144 MHz
96 kHz
Film sound, recording studios
12.288 MHz
192 kHz
Top end, highest quality
24.576 MHz
Table 1
You will note that Table 1 starts at 44.1 kHz, the sampling rate for consumer CD's, digital audio track on consumer camcorders, and many other consumer applications. The quality of this sampling rate is considered to be slightly better than the performance of FM radio broadcasts.
The sampling rate is not the bandwidth of the signal running down the cable. The bandwidth is determined by a number of factors. First is the digital word' size, the number of bits in each section of data. The maximum allowed in the AES spec is 32-bit words. The second factor is that the words can carry one or two channels of audio for every bit-stream. This is why, for instance, on your home hi-fi receiver, it has a single RCA connector labeled "digital audio". Than single connector will carry two channels of audio. The final factor is that, for twisted pairs, the AES chose a system called bi-phase' which mean, in the twisted pair, it doesn't matter which wire goes into which pin as long as you have the correct two wires going into the correct two pins.
What this all means is that you have to take the sampling rate and multiply it by 32 (word size) and again by 2 (2-channels) and again by 2 (bi-phase). Or you can just multiply the sampling rate by 128 (32x2x2). That formula will then tell you the bandwidth of the signal. And, as you can see in Table 1, the actual signal bandwidth on a digital twisted pair, is way beyond the 20-20 kHz of analog audio. In fact, it starts at almost 6 MHz. The AES committee soon realized that this bandwidth meant that the wavelength of even a consumer digital audio bitstream was easily reached as can be seen in Table 2.
SAMPLING RATE
BANDWIDTH
WAVELENGTH
QUARTER-WAVE
44.1 kHz
5.6448 MHz
53m
13m
48 kHz
6.144 MHz
49m
12m
96 kHz
12.288 MHz
24m
6m
192 kHz
24.576 MHz
12m
3m
Table 2
The committee realized that this mean they had to choose a specific impedance for these twisted pairs. The ideal impedance, one with the lowest loss, would be 150. But this would make a huge cable, one that would never fit into an XLR (the desired connector). So the AES committee reduced the effective distance and changed the impedance to get the size down to something much closer to the size designers and installers were used to in the old analog world. And this impedance turned out to be 110.
AES3 also spells out how far one can go on a given twisted pair. If you feed an AES digital signal of 2 volts in, and a 0.2 volt (200 mV) signal out, that's as far as you can go. This distance is also affected by the resistance (i.e. size) of the wires, so different gage wires will go different distances. But since we're well within the quarter wavelength distance, the source, the cable, and the destination (load) impedance should all be 110. This is now a "transmission line" where we must match impedance. Table 3 shows these distances.
GAGE
6 MHz
12MHz
25MHz
26 AWG
248m
193m
145m
24 AWG
337m
267m
198m
22 AWG
469m
381m
309m
Table 3
Table 3 might surprise a lot of people who were always told "digital audio signals can't go very far". In some cases (if you read the previous installment on analog audio), these digital signals can go farther than analog audio signals!
You have a number of choices to run these digital signals. Of course, there are AES twisted pairs. But there are at least three variations on that theme aloe to choose from. Then you have another standard AES3-id, that allows you to run these digital audio signals down coax cable. We'll visit all of these.and morein our next installment!
