Thankfully a complete understanding of compression
rates and other aspects is not required to use
minidisk or for that matter any other medium for
recording. If it was I suspect the number of
recordists would be miniscule. This is not to suggest
that an understanding is not desirable just not
essential. I do however applaud those who do
comprehend fully the technicalitys and take the time
to explain them to us lesser mortals.
Tony Baylis.
--- Bret <> wrote:
> 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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