Telex Integrated Signal Processor ISP-100 - Manual (Page 97)

8-27
Processing Components - Dither
Dither
Figure 8-13 Dither component.
Dithering (see Figure 8-13) is used when shortening high resolution
digital audio data words to a lower resolution; for example, when sending
24-bit ISP-100 audio data to 20-bit D/A converters, or directly to a 16-bit
Digital Audio Tape recorder through a digital interface. Simply truncating,
or discarding, the unused bits results in audible artifacts that are
generally objectionable. Dithering does not eliminate these artifacts, it
changes their characteristics so that they are less noticeable to the
human ear.
If, after reducing the number of bits in a signal, the original high-
resolution (more bits) signal is subtracted from the final lower-resolution
(fewer bits) signal, the difference is called the error signal:
(Final Signal) - (Original Signal) = (Error Signal)
Some very simple algebra shows that the final signal can be thought of
as the sum of the error signal and the original signal:
(Final Signal) = (Error Signal) + (Original Signal)
When audio data words are shortened by simple truncation, it turns out
that the error signal is correlated with itself and with the original signal.
This simply means that the error signal is not pure white noise, and that
the frequency and amplitude of the error signal is in some way related to
the frequency and amplitude of the original signal.
If noise with some very specific amplitude and spectral characteristics is
added to the original signal before truncation, then the correlation is
destroyed and the error signal sounds like low level noise instead of
tones. Even though the added noise actually raises the noise floor in the
final low-resolution signal, it sounds better because human hearing finds
low level broadband noise to be much less objectionable than spurious
tones.
This addition of noise before truncation is called dithering. The noise that
is added is called the dither signal.
In summary, dithering is a way of making digital audio better by making it
worse.
Bit Level
In the ISP-100 the bit level of the dither signal is selectable as whole