Model VR240 Audio Logging Recorder
4-4
January 2000
Harmonic distortion by itself is that part of the output which is harmonically related to the input
sine wave with the fundamental subtracted. Distortion is visible on an oscilloscope if the output
waveform doesn’t look like a sine wave. A common and severe form of distortion is clipping,
which is visible as a flattening of the peaks of an audio waveform. In analog systems, clipping
may be rounded and may actually sound pleasant up to a point. In digital systems, clipping
means that the input to the analog-to-digital converter is above the maximum; the largest
sample value is not large enough to serve as a measurement of the input signal. When
reproduced, the waveform from a clipped digital signal is perfectly flat where the peaks could
not be properly sampled, resulting in a very unpleasant sound.
4-8.
WOW AND FLUTTER.
In an analog recorder, wow and flutter are caused by variations in tape speed due to lack of
mechanical precision, and they get worse as the absolute speed of the tape decreases. To
avoid unacceptable wow and flutter, the tape mechanism must be kept meticulously clean and
aligned. How much wow and flutter can be tolerated depends on the application. If the goal is to
listen to a section of a recording and transcribe it, significant amounts can be tolerated. If the
goal is to reproduce and analyze critical signals, even tiny amounts can seriously degrade the
signal.
Because digital recording uses electronic clocking of its input and output, and electronic signals
are much more precise than mechanical devices, a digital recorder like the VR240 will have so
little wow and flutter that it will be unmeasurable. Digital audio is stable as long as the sampling
interval is precise while recording and playing. Once sampling is done, steady timing is not
important until the audio must be reproduced. During the recording process, each channel is
sampled every 125 microseconds. Samples from all channels are accumulated in memory until
a certain number is reached, at which time the accumulated samples are copied to tape at high
speed. Playback is the reverse: every few seconds a large number of samples is copied from
tape into memory; samples are then taken from memory, separated into the original channels,
and converted to analog one at a time every 125 microseconds. Think of a bucket of water with
a small hole in the bottom: water (samples) will drip out of the bucket (memory) at a fairly
constant rate whether there is one inch or several inches of water in the bucket. This type of
operation explains why the tape in the VR240 does not move steadily.
4-9. CROSSTALK.
In an analog machine, channel separation is achieved by recording different channels on
different areas of the tape, called tracks. There is always a compromise in tape head design:
the closer together you put the heads, the more tracks you can get on a tape, but the greater
will be the crosstalk between channels.
The VR240 has only one physical “track”. Along with the number representing audio samples,
the VR240 adds time and channel information so that when the tape is played back the samples
are sent to the proper output channels The only crosstalk in the VR240 is a result of proximity
of electronic circuitry, and is easy to control.
