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Analog
to Digital Conversion and Digital Audio Signal Transfer
- In order to perform acoustic analysis on
recorded speech data or to deliver audio on-line, the audio
signal has to be converted into a digital audio file format,
such as Wav or Aiff. Analog recordings have to be digitized
and digital recordings need to be transferred to a personal
computer via a digital audio file transfer interface. This
is an important, yet often underestimated, stage in the process
of preparing audio data for analysis.
The
main goal of A/D conversion (digitization) is to obtain the
best possible digital representation of the original analog
waveform. Without going into too much technical detail of
the digitization process, one should choose a sample rate
that will capture a broad range of frequencies and a bit-depth
that will allow a wide dynamic range and a negligible amount
of quantization noise. These goals can be achieved by means
of a premium-quality, stand-alone A/D converter operating
at the sample rate of at least 48,000 Hz and a 24-bit resolution.
It is absolutely crucial not to use a PCI multimedia sound
card, as they are built from inferior-quality electronic components
and, more importantly, allow electrostatic noise and distortion
to leak into the captured acoustic signal:
Spectrum
of typical electrostatic noise generated by computer circuitry.
The
A/D converter, such as Lucid AD 9624, should offer a variety
of sample rates, oversampling, high quality anti-aliasing
filters, and AES/EBU and S/PDIF digital outputs. Both AES/EBU
(Audio Engineering Society/European Broadcasting Union) and
S/PDIF (Sony/Philips Digital Interface) are fairly common
on high-end digital audio devices. In addition, S/PDIF is
used on a variety of consumer-level products, such as CD players,
minidisk players, etc. It is also a common interface used
on PCI digital I/O cards, which is why it is probably a better
choice for most digital audio transfer applications.
The
analog playback device (such as TASCAM 122 mkIII) should be
connected to the A/D converter. One should make sure that
the output levels on the tape deck match the input levels
on the A/D converter. It is recommended to use balanced XLR
line level interface (+24 dBu min. gain, +7 dBu max. gain,
65k ohm impedance). If the tape deck does not have this kind
of output interface, a signal level transformer (such as Ebtech
Line shifter PHOTO>>) and a pre-amplifier should be
used.
The
A/D converter needs to be connected to a PCI (though USB and
FireWire are becoming common) digital audio I/O card (such
as Midiman Delta DiO 2496 via a S/PDIF interface). The digital
I/O card should be selected as the recording interface in
the audio recording software (such as Sonic Foundry Sound
Forge 5.0 on a PC or BIAS Peak VST on a Mac). The digital
audio signal should be captured with this software and saved
either as Wav (PC) or Aiff (Mac) file at the sample rate and
bit depth that the A/D converter was set to. It is also possible
to capture digital audio signal directly into acoustic analysis
software, such as CSL or Praat, though it is not recommended
due to the fact that specialized recording and processing
software offers considerable more control over the incoming
signal. It should also be mentioned that USB Pre may be used
as a high-quality, stand-alone A/D converter.
In
this case the digital audio signal is transferred to a PC
via the USB interface, which eliminates the need to install
a separate PCI digital I/O card and makes it possible to capture
digital audio on a laptop. In addition, USB Pre has a pair
of tape-level inputs, to which a cassette deck can be directly
connected.
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