Per http://www.minidisc.org/aes_atrac.html
the data size required to record to minidisc is about 1/5th the size
required for cd quality, 16bit 44.1khz sampling.
The method of compression is not as simple to explain. More bits of
storage space are allocated to 'more important' portions of the sound
and fewer bits to 'less important' portions as defined by our
understanding psychoacoustic principles. These principles involve our
hearing threshold as a function of frequency, masking of one sound by
another, and 'critical bands', 'the idea that the ear analyzes the
audible frequency range using a set of subbands'.
Per the above link:
'It is clear that the critical bands are much narrower at lower
frequencies than at high frequencies; in fact, three quarters of the
critical bands are located below 5 kHz. This indicates that the ear
receives more information from the low frequencies and less from higher
frequencies.'
'ATRAC uses psychoacoustics not only in the bit allocation algorithm,
but also in the time-frequency splitting. Using a combination of
subband coding and transform coding techniques, the input signal is
analyzed in nonuniform frequency divisions which emphasize the
important low-frequency regions. In addition, ATRAC uses a transform
block length which adapts to the input signal. This ensures efficient
coding of stationary passages without sacrificing time resolution
during transient passages.'
Some frequency bands are considered more important. Bit allocation is
determined based on 'time-frequency analysis'
'First, the signal is broken down into three subbands: 0-5.5 kHz,
5.5-11 kHz, and 11-22 kHz. Each of these subbands is then transformed
into the frequency domain, producing a set of spectral coefficients.
Finally, these spectral coefficients are grouped nonuniformly into
BFU's.' (block floating units)
'The analysis block decomposes the signal into spectral coefficients
grouped into Block Floating units (BFU's). The bit allocation block
divides the available bits between the BFU's, allocating fewer bits to
insensitive units. The quantization block quantizes each spectral
coefficient to the specified wordlength. '
bret
--- Vicki Powys <> wrote:
> I have recently been asked:
>
> What is the compression rate for currently available minidisc?
>
> Any quick and simple answers out there?
>
> We were discussing the merits of the newer consumer minidiscs eg
> Sharp MD
> DR7 (apparently unavailable in Australia) and Sony MZN10 which seems
> to be
> the only current Sony model available in Australia that can record
> via a mic
> input.
>
> Vicki Powys
> Australia
>
>
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