Townshend Computer Tools DAT-Link - 6.6 Specifying the Recording Duration; 6.7 Sampling Rates

6.8 Skipping Silence 65
6.7.1 DAT Sampling Rate
The DAT-Link can accept audio input sampled at any rate between 25,000 and
55,000 samples per second. Since the data is generated by the digital audio device,
this rate is not controlled by the DAT-Link . Most DATs are recorded at 48,000
samples/second while CDs are usually recorded at 44,100 samples/second.
6.7.2 File Sampling Rate
If the le stored on the computer is not at the same sampling rate as that which
will be used by the DAT machine, a sampling-rate conversion op eration must be
performed. This is done in real time by the DAT-Link using its built-in digital
signal processor. Toinvoke a conversion, you need only specify the sampling rate
of the le using the -s option. As an example, assume you wish to store only the
right channel of the incoming audio at a sampling rate of 8,000 samples/second.
The appropriate command would b e:
narecord -oright -s 8000 file
The -oright option selects right-channel only, and the -s 8000 means the le
should be sampled at 8,000 samples/second. To p erform this command the DAT-
Link will perform a digital resampling op eration which will result in some low-pass
ltering of the audio signal. See Chapter 7 for more information on the eects of
this conversion.
If the -s option is not used, then narecord will store the le at the same rate as
the data was presented to the DAT-Link , without performing any sampling rate
conversion. The user must be aware of the actual sampling rate that was in use at
the time to correctly play back les recorded in this way.
6.8 Skipping Silence
Often when recording there will be a silent perio d b efore the actual material of
interest. This may be due to the time involved in starting the tape, a pause before
beginning a live recording, or a silent lead-in on tape. To minimize disk usage,
the narecord program has an option, -q , which instructs the DAT-Link to skip
this silence and b egin transferring data when a threshold level is exceeded. This
threshold is xed at a sample value of 128 where the maximum possible is 32767.
When recording without sample rate conversions, the rst sample that will b e stored
in the output le will b e approximately 10,000 samples b efore the rst sample ab ove
threshold. If sample rate conversion is active, then the number of samples before
the threshold is reached will depend on the conversion parameters, but will usually
be at least 100 samples.