Apogee AD-1000 Operating Manual page 52

AD-1000 Operating Manual
rate tempo and add faster multiples many times higher than the drummers sixteenth-note example. Now imag-
ine what would happen to the drummer's playing if we put slight, random variations in his click track reference.
The drummer would try to follow the changing tempo but because the changes were unpredictable, he would
overshoot the click tempo as it moved up and down. The random click track variations around a perfectly steady
tempo could be called tempo jitter. The poor drummer ends up with worse jitter in his timing unless he can
ignore the small changes and play to the average.
The problem of interconnects affecting the sound can be traced to jitter in the timing of the digital to analog
playback. Each time digital audio timing is passed through additional circuits, it picks up slight variations around
the original perfect timing. The amount of timing jitter added through successive stages depends on the type
of circuits. Inside products, different computer logic families used for digital calculations add varying amounts
of jitter. Noise on power supplies and grounds, nearby clocks with similar harmonics, AC power and external
interference can all add jitter to perfect timing. Some of it is random and some has specific frequency content.
When the internal timing is passed to another device over an interconnect, different types of connections add
more or less jitter. A short AES/EBU connection over high quality digital audio cable – such as Apogee's Wyde
Eye 110 AES/EBU cable – will pick up less jitter than the same signal run through a length of microphone
cable, XLR connectors and patch bays. A S/PDIF coaxial wire connection (especially one made with Wyde Eye
75 cable) will be cleaner than the consumer "TOSLINK" optical version, at least partially because of the slow-
er response time of the optical transmitter and receiver.
When the circuits in digital to analog converters (D/A's) recover the timing, they are often negatively influenced
by the jitter picked up along the way, much like our miserable drummer trying to follow the varying click track.
When the recovered timing starts to wobble around as it tries to track the jittery input, it modulates the ana-
log sound coming out of D/As, causing all sorts of subtle negative effects such as changes in the stereo image
and tonal quality. An interesting source of jitter in AES/EBU digital interconnects is due to the changing sam-
ples and subcode information. A 1kHz digital audio tone causes 1kHz jitter.
Different interconnects do not sound different if the timing circuits of the reference D/A are designed to ignore
any jitter and the samples are correctly transmitted. Manufacturers can claim low jitter circuitry – although it's
only a relative claim, as at the moment there are no accepted standards for jitter measurement for digital audio.
Jitter also has a big influence on the quality of analog to digital converters with very similar side effects, which
unfortunately are there forever after.
Interconnect Formats Before AES/EBU
Sony SDIF Interface
The Sony SDIF interconnect is a good example of a basic digital connection between two digital audio units.
SDIF format interconnects are found on the Sony 1610 and 1630 processors (used to generate CD masters on
U-Matic video cassettes) and many Sony professional digital audio products. The original SDIF and newer SDIF
II both use two connections for mono transmission and three for stereo. Each channel is transmitted on its own
separate connection along with one word sync connection for any number of audio channels. The main differ-
ence between SDIF and SDIF II is unbalanced operation with SDIF and balanced for SDIF II. The 3324 and 3348
digital multitrack recorders have balanced connections which require electronic translation to unbalanced for
connection with the SDIF Input/Output on 1610 and 1630 processors. Although the Word Sync transmits elec-
trical pulses at the sample rate for synchronizing the sample transfers, the same information is also contained
within each audio channel, so any timing delays between the word sync and channels (due to different wiring
lengths) will not corrupt the audio. The SDIF format can handle up to 20-bit samples and includes emphasis
identification. The Sony word sync is a symmetrical square wave. The audio samples are transmitted with the
most significant bit (MSB) first, the same sequence we read out our familiar decimal numbers.
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