On 28/04/14 13:47, Jim Lesurf wrote:
In article , Andrew Gabriel
wrote:

Struggling to remember, but ISTR a tape lasts 30 mins at 7.5"/sec.

Erm. That would depend on the length of the tape. Which in turn pun
depends on the size of the spools and the tape thickness.

The frequency response at 7.5"/sec looks to have been up to 15kHz. In
thinking of this digitally, that's a sampling rate of 30kHz as per
Nyquist.

Chances are that a decent R2R will record above 15kHz at 7.5ips. Depends on
the tape, heads, etc. Unlike LPCM there's not sharp cutoff.

Even a decent home machine like the old Tandberg 3000 would do 40 - 20k at
+/-2dB running at that speed. And could record higher frequencies provided
they were're high level and you accepted a drop in output. The unweighted
noise approached -60dBref and you could also record above ref for
mid-frequency signals. And I'm sure there were better decks around.


I've no idea what the dynamic range is, but let's guess it's at
least 8 bits worth (it's probably more). That works out at 30kbytes/sec,
or 60kbytes/sec for two-channel (stereo). So for a 30 min tape, that's
just over 100Mbytes.

Its likely to be more than 8bits worth, but again depends on the tape,
deck, etc. And it isn't easy to estimate because the response and noise
floors aren't flat in-band. Nor is the max signal level limited by an
abrupt ceiling. Instead by a rise in distortion that depends on the details
of the signal being recorded.

Plus, of course, a tendency later on the use Dolby level compansion which
further complicates the issue. And ignoring the detail of if you are
thinking of a full/half/quarter track for mono or stereo. I'm assuming
the usual home machine width and format. :-)

So 20kHz bandwidth and about 60dB range would be a closer guide, but
probably still not give a really representitive value for some 'Mbytes'
result.


You are pushing your luck with that on any but seriously pro kit. Th
pre-emphasis on the treble means you only have a few db range above 10kHz.

Which is why disks and data tapes use a completely different encoding
system.

Id say a fast tape - 15 ips or more would net you perhaps 60dB at 2Khz
bandwidth if reliability needed.

Tape was truly AWFUL.

CD MUCH better.

Id doubt that a tape was capable of delivering much more than a good
modem over a phone line, 64kbps.

Bandwidth and S/N are similar.
so 8KB/s

You only need to look at the mess that videotapes were on all but
massively expensive kit at stupendous speeds to see that getting to the
Mbps was almost impossible

Of course the wider the tape the better..




Jim



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