Operator`s manual

Model VR240 Audio Logging Recorder
4-2
January 2000
audio signal is like using very grainy photographic film: the image (audio) becomes very fuzzy
(noisy).
In digital recording, as used on the VR240, the audio signal is measured at regular intervals and
converted to numbers. Instead of relying on the amount of magnetization on the tape to
accurately represent an audio signal, digital recording relies on the magnetization to represent
only two values – zero and one – needed to support binary numbers. Using the comparison
with photography again, we can say that with digital recording we can tolerate very “grainy film”
since we only need to distinguish light (one) from dark (zero). As long as we can store and
reproduce the ones and zeros without error, the quality of the resulting audio depends on how
we use the numbers.
To maximize sound quality in a digital recorder, you measure the audio signal very precisely
and very often; in other words, you use a lot of tape. To maximize recording time, you cut back
on accuracy and don’t measure so often. Audio quality is traded off, but with digital recording
there is 1) more control, 2) no degradation when making digital copies, 3) no wow or flutter, and
4) no crosstalk from tape “tracks” too close together.
4-4. AUDIO QUALITY CONSIDERATIONS AND TRADEOFFS.
Audio quality in a tape recorder is usually determined by several measurable quantities:
•
Frequency Response
•
Dynamic Range
•
Signal-to-Noise Ratio (SNR)
•
Total Harmonic Distortion plus Noise (THD+N)
•
Wow and Flutter
•
Crosstalk
4-5. FREQUENCY RESPONSE.
Frequency response is the range of frequencies over which a machine can record and
reproduce a sine wave such that the reproduced signal has the same relative amplitude as the
input signal. “High fidelity” audio equipment should have a frequency response of 20Hz-20kHz
±1dB. It is commonly agreed that a frequency response of about 200-3000 Hz is necessary to
faithfully reproduce human speech.
In an analog recorder, frequency response is directly proportional to tape speed, and is also
related to the type of tape and the design and condition of the tape heads. It is truly remarkable
how good a standard audio cassette deck can sound considering the tape speed is only 1 7/8
inches/second and that the format was designed for voice recording. Professional analog tape
recorders used for music run at 15 or 30 inches/second.
In a digital recorder, frequency response is directly proportional to sampling rate, or the number
of times per second that the analog signal is converted to a number. To the extent that each
number requires a certain amount of space on the tape, the frequency response of a digital