"Andrew Gabriel" wrote in message
...
In article ,
Bill Wright writes:
Katie, who is 11, was helping me in the workshop today. She told me how
some of her class been in trouble for filming the teachers covertly. Of
course I told her how, in 1965, I'd made a sound recording of our maths
teacher, with the class deliberately winding him up just to make it more
fun. I then found myself trying to explain about reel-to-reel tape
recorders. I could see that Katie just couldn't grasp the concept.
Finally she asked, "But how much memory did it have?"

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

Today, that would be mpeg compressed on a personal player, so perhaps
just 50Mbytes (won't get as good compression as modern digital recordings,
because original data is already at a reduced resolution).

So best answer is that the tape player has no memory, but each tape
corresponds to perhaps a 50Mbyte SD card today (way smaller than
anything you can buy today)

My estimate would be 30 minutes at 3 3/4 inches per second, mono, 10 kHz
frequency response, signal to noise ratio 40dB.

That converts to 1800 seconds at 20 k samples per second. 40 dB would be
about 8 bits, but 8 bit audio sounds rougher than tape, so I'd guess 10 or
12.

10 bits x 20k x 1800 = 360 Mbits = 45 MBytes.

Has she ever seen a VHS cassette? They come out at 1000 times more.
3 hours, 2.5 MHz frequency response, 30 dB s/n, converts to

6 bits x 5000k x 10800 = 324 Gbits = 40 GBytes.




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