Sound Devices 702 - Manual (Page 23)

702 User Guide and Technical Information
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measure (exponent of 2), so as bit depth increases, the amount of data it represents increases expo-
nentially. The majority of field recording is done with 16-bit audio, therefore, each sample is rep-
resented by a digital word of 2^16 (65,536) possible values. 24-bit audio has a word length of 2^24
(16.7 million) possible values per sample.
The 702 has 24 bit analog-to-digital converters. To obtain 16 bit recording the 702 can be set to dither
the 24 bit digital signals output from the analog-to-digital converter to 16 bit. The 702 uses a pro-
prietary pseudo-random dither routine for accurate bit rate reduction. Dither can be defeated in the
user menu. Without dither, 24 bit audio is truncated to 16 bit, meaning the least significant 8 bits are
discarded.
Once a file is recorded its sampling rate and bit depth can not be changed in the recorder. The 702 does
not perform sample rate conversion or bit depth changes. File conversion must be done in another en-
vironment, such as an audio workstation. Alternatively, a real-time analog transfer is often performed
instead of sample rate conversion.
Word Clock
Stable word clock is fundamental to a high quality digital audio signal. The 702 uses a highly-stable
crystal to generate its internal word clock. The 702 can clock external devices from its word clock and
accept external clock sources for recording.
The 702 ignores external clock, both AES and word clock, during playback.
Clock Master
When sending digital audio to several devices, one unit is designated as the word clock master and
the others as slaves. Generally, the device with the analog-to-digital converter is designated as the
word clock master.
The 702 can function as an analog-to-digital converter and can be used as a master word clock
source. Slaved devices will derive their word clock timing from either their digital audio inputs, S/
PDIF or AES/EBU, or through their word clock input connection. As a word clock master the 702
generates word clock whether or not audio is sent.
Clock Slave
When using an external digital preamplifier connected to the 702 inputs, the recorder can derive its
clock signal from the AES (S/PDIF) stream (it will slave to the external device), or the external device
can be slaved from the 702 (if the external device has word clock input or accepts clock from the
702’s digital output). For example, if you are using a wireless receiver with a digital output, it may
not have an external word clock input, and must be the word clock master.
If digital audio is connected to the 702 from more than one digital device, you must word clock the
sources to the same clock, otherwise variations between the sources will render their signals unus-
able.
If the 702 is slaved to external word clock, be certain that the source is stable. Loss of the word clock
signal during recording can cause the 702 to revert back to its internally set sampling frequency. If
this occurs, the portion of the file recorded after the loss of word clock may not play back at the prop-
er speed. For reliability, set the 702 to the same sample frequency as the word clock source. Loss of
the word clock signal in this case will likely cause a glitch in the file, but the file may still be usable.