Re: Did Commodore cheat with the quad density floppies?

From: Mia Magnusson <mia_at_plea.se>
Date: Sat, 5 Jan 2019 22:13:27 +0100
Message-ID: <20190105221327.00003d9c@plea.se>
Den Fri, 4 Jan 2019 22:51:18 +0000 skrev smf <smf@null.net>:
> On 04/01/2019 20:07, afachat@gmx.de wrote:
> >   But it is recorded at the double frequency.
> >   So in total, encoding, plus
> > double frequency, it is more efficient than MFM.
> 
> MFM and GCR aren't exclusively used with floppy disks, the efficiency
> of the encoding is the number of bits in vs the bits out. Not
> anything hardware specific.

I've never ever heard of any useage of MFM or GCR on non-analogue
transmissions.

It doesen't matter if the data is stored on a floppy, a hard disk, tape
or some other media, or even transmitted on a wire (with a limited
bandwidth).

With true analogue storage, there is a difference of the highest
frequency of a reasonable readable signal, and how exact the timing
between two pulses can be detected. Some properties of the media affect
both, while some (for example width of the head) only affects the
maximum frequency but not the precision of the timing.

When signals (that are intended to be used as analogue information) get
digitized they are usually digitized at a sampling frequency slightly
higher than the maximum signal frequency. That is usually good for for
example audio, while it's unusable for composite video as you lose the
phase information.

You could say that MFM also uses the phase of the signal while GCR
doesen't, and therefore the same media can be used for MFM at double
the clock frequency than GCR, as the doubled clock frequency for MFM is
only used for making phase shifts possible but not actually a higher
signal frequency.
 
> Look at that amiga software I linked to which uses 19 gcr sectors per
> track (9728) rather than the usual 11 mfm sectors (5632 bytes), it's
> obviously running at the faster rate but using a more efficient
> encoding than normal.

It requires HD media and achieves a worse result than MFM (Amiga MFM 22
sectors per track).

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Received on 2019-01-05 23:01:06

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