Mary C. Gruszka /
01.25.2006 12:00 AM
The Importance of Spec Design, Part 2
Last time we started to take a critical look at audio specifications by using as an example, a particular audio mixer with both digital and analog inputs and outputs. Most of the inputs and outputs on this console are analog, while only a few are digital, namely program outputs, aux sends and returns, and two-track inputs.
The goal of this exercise is to show how to examine specifications very carefully, even reading between the lines, to decipher what information manufacturers tell us, and also, quite revealing, what information they leave out.
Let's return to our audio console and look at a few more specs.
MORE THAN 1 dB
Monitor outputs are -10 dBs. Headphone output of the mixer is -26 dBs at reference level, but it doesn't specify what that reference level is. On the plus side, the spec does say that the stereo phone jack wants to "see" an 8 ohm load; that means no 600 ohm headsets.
But what is a dBs? I see that term in many specifications. From the way it is written, the 's' in dBs could be interpreted as an abbreviation for the reference level. But what reference? For dBV the 'V' means that 1volt is the reference. For dBm the 'm' means that 1 milliwatt is the reference. But 's'?
My guess is that in this spec, dBs means nothing more than the plural of dB, as in one dB, two dBs. But that tells us nothing useful. A decibel simply represents a ratio of two parameters, voltage, current, power, sound pressure level, to name just a few.
If one voltage is 6 dB higher than another voltage, we would know that the first voltage was twice the second, but we wouldn't know what either voltage is. It could be 2 volts compared to 1 volt, or it could be 400 volts compared to 200 volts. Either way, the ratio is 2:1 or 6 dB.
Now back to our monitor output. Since we aren't given the reference level, all we know is that this output is 10 dB lower than something, but we don't know what, and we can't calculate the actual voltage output.
At this point let's try to add some gear with digital audio I/O to our system. Perhaps a DVD recorder.
I checked one particular company's Web site for pro gear (that company also makes consumer equipment), clicked on a link for broadcast/professional video, chose DVD recorders, and got a list of model numbers, again under the heading of broadcast and professional video.
I decided on a combination recorder that has DVD, mini-DV, and a hard disk recorder. The feature details say that this device has excellent digital audio and that a variety of audio formats can be recorded including MiniDV in PCM 48 kHz and linear PCM (no sampling frequency given for this mode). But what about connections?
For that I had to open a .pdf file and study a fuzzy picture of the rear connector panel to realize that there weren't any professional audio connections at all! It looked like the digital audio output was on an RCA connector (the very limited audio spec noted that the output was coax). Chances are that the output was S/PDIF, the digital audio format used more often for consumer products, not the professional AES/EBU. In addition, the listing of inputs and outputs didn't indicate a digital audio input.
There were two other sets of RCA connectors for audio in and out, but presumably, these were analog connections, although not specifically stated. With RCA connectors, these I/Os would be inherently unbalanced, which are not professional connections in my book.
While I don't feel that this device could be categorized as a broadcast or professional product, in any given project, such types of devices often need to be integrated into the system. How could we handle this DVD player?
I would first confirm with the manufacturer that the digital audio output was indeed a S/PDIF connector, and also try to get a clearer picture of the rear connector panel. If indeed the output is S/PDIF, then I would use a S/PDIF-to-AES converter, since our audio mixer has AES inputs. If I could find a converter that had a BNC AES output, so much the better, since our original requirement was to distribute AES via 75 ohm coax. However, in the more likely case that the converter had an AES XLR connector for the output, then we would need a 110 ohm to 75 ohm transformer. For the analog side of this DVD player, we could take two routes. One is to keep the signals in the analog domain and use unbalanced/balanced converters to go in and out of the analog side of the audio mixer. For this particular console with more analog I/O than digital, this would probably be the better approach.
TIMING IS AN ISSUE
But if we had a different digital audio console or wanted to feed a digital output from our example mixer to the analog input of the DVD player, we would need two converter boxes--one, a digital-to-analog converter, which, being a professional unit, would have balanced outputs, and then an analog balanced/unbalanced converter.
Timing is also an issue in digital audio systems design. All devices with digital audio I/O must be synchronized together, and for digital audio gear working in a video environment, everything should be locked to video house reference (color black).
The specs for this DVD player don't indicate a word clock reference input, but maybe a better picture of the rear panel would show this connector. If there isn't one, and we still need to use this unit for its feature set, then the digital output or the output of the S/PDIF-to-AES converter may need to be run into a sample-rate converter that is locked to house reference. Otherwise, pops and clicks could result when feeding the output of this machine into the digital audio input of another device.
If any of the converters mentioned here aren't frame-mounted, make sure you have extra power outlets for all the wall warts.
As can be seen, the design starts to get more complicated when integrating devices with consumer connections into a professional system. Reading specifications closely and critically will give an indication as to how easy or difficult it will be, and what external conversion devices may be needed. If the specs don't give the answers, contact the manufacturers.