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You have to be careful about the medium's physical characteristics, and which way you want to store data. The commercial audio casette system (magnetic tape, and read-write circuitry together) had a bandwidth $B$ of about 20 kHz. And I guess its SNR is about 60 dB. This can be an under or over estimation, if you consider what I've said at the beginning; CD-quality audio. Now to be honest, Casettes do not provide CD-quality (low-noise metalic casettes with Dolby type-s noise reduction is said to achieve a CD quality though), as there is some characteristic high frequency hiss noise in those commercial casettes, especially on the lower quality tapes. Therefore I will take 60 dB as an average estimate. Also note that this SNR depends on the frequency, as the noise increase by frequency, but I will ignore it too.

And assuming a two channel stereo recording this yields about $800 ~kbps$ channel capacity. Indeed, considering that CD had a much better SNR about 96dB, it has a channel capcity of $1.411$ Mbps. Then the commercial audio casette had an equivalent data rate about half of the CD medium. In one hour this makes about $360$ MB (Mega Bytes) of data.

The capcity is there, but how can you utilize it?

The audio CD, for example, uses a very small tiny pattern on the circular track lines. The bits are placed as very short and very thin reflective vs nonrelective sections along the track lines that spiral from the innermost circle to the outermost, just like in Vinyl. There is no noise, but distortion due optical reflection, diffraction, and refraction, mechanical disturbances, which limits the resolution at which you can place or read those tiny dots on the circular tracks.

This kind of M-ary encoding is very well utilized in Satellite communication, where 256-QAM is used to take advantage of (relativlely) low noise transmission channel...

Coming to your Kansas City Standards, it's not about the magnetic medium capacity, but about the commercial phone line allowed channel capacity which was quite limited to about $3600$ Hz back then...

So the conclusion is; if you can afford a suitable encoding technique, in principle, you can store about 360 MB of data into a standard commercial magnettice type cassette. There are low noise (high SNR) casette types (such as Cr or Metal) available which would increase the capacity. Or at least you can roll a longer reel to increase it. Whether you can accomplish this or not is another practical concern though.

You have to be careful about the medium's physical characteristics, and which way you want to store data. The commercial audio cassette system (magnetic tape, and read-write circuitry together) had a bandwidth $B$ of about less than 20 kHz. And I guess its SNR is about less than 60 dB. This can be an under or over estimation, but if you consider what I've said at the beginning; CD-quality audio, now to be honest, commercial audio cassettes do not provide CD-quality audio, (low-noise metal cassettes with Dolby Type-S noise reduction is said to have achieved a CD quality though), as there is some characteristic high frequency hiss noise in those commercial cassettes, especially on the lower quality tapes. Therefore I will take 60 dB as an average estimate. Also note that this SNR depends on the frequency, as the noise increase by frequency, but I will ignore it too.

And assuming a two channel stereo recording this yields about $800 ~kbps$ channel capacity. Indeed, considering that CD had a much better SNR about 96dB, it has a channel capcity of $1.411$ Mbps. Then the commercial audio casette had an equivalent data rate about half of the CD medium. In one hour this makes about $360$ MB (Mega Bytes) of data , as an optimistic upperbound, as the comments indicated.

The capacity is there, but can you utilize it?

The audio CD, for example, uses a very small tiny pattern on the circular track lines. The bits are placed as very short and very thin reflective vs non-reflective sections along the track lines that spiral from the innermost circle to the outermost, just like in Vinyl. There is no noise, but distortion due to optical reflection, diffraction, refraction, and mechanical disturbances, which limit the resolution at which you can write or read those tiny dots on the circular tracks.

This kind of M-ary encoding is very well utilized in Satellite communication, where 256-QAM is used to take advantage of (relativlely) low noise transmission channel.

Coming to that Kansas City Standards, it's not about the magnetic medium capacity, but about the commercial phone line allowed channel capacity which was quite limited to about $3-4$ kHz back then...

So the conclusion is; if you can afford a suitable encoding technique, in principle, you can store about less than 360 MB of data into a standard commercial auido cassette. There are low noise (high SNR) casette types (such as Cr or Metal) available which would increase the capacity. Or at least you can roll a longer reel to increase it. Whether you can accomplish this or not is another practical concern though...

Of course, magnetic mediums have different uses, types and qualities. And I think, one thing to mention at the beginning is that, the commercial casette have a tendency of losing its quality. This is especially true with music recording, where high-fidelity is lost after a few playbacks. Such an untrustable charcateristic makes them a bad choice for reliable data storage at high rates. Hence possibly lowering their available capacity, in exchange of increased reliability and durability.

Of course, magnetic mediums have different uses, types and qualities. And I think, one thing to mention at the beginning is that, the commercial casette have a tendency of losing its quality. This is especially true with music recording, where high-fidelity is lost after a few playbacks. Such an untrustable characteristic makes them a bad choice for reliable data storage at high rates. Hence possibly lowering their available capacity, in exchange of increased reliability and durability.

But, you have to be careful about the medium's physical characteristics and which way you want to store data. The commercial audio casette system (magnetic casette and recording-reading circuitry together) had a bandwidth $B$ of about 20 kHz. And I guess it's SNR is about 60 dB. This is an underestimation, if you consider what I've said at the beginning; CD-quality audio. Now to be honest, Casettes cannot provide CD-quality, as there is some characteristic high frequency hiss noise in those commercial casettes. Therefore I will take 60 dB as an estimate. Also note that this SNR depends on the frequency, but I will ignore it too.

You have to be careful about the medium's physical characteristics, and which way you want to store data. The commercial audio casette system (magnetic tape, and read-write circuitry together) had a bandwidth $B$ of about 20 kHz. And I guess its SNR is about 60 dB. This can be an under or over estimation, if you consider what I've said at the beginning; CD-quality audio. Now to be honest, Casettes do not provide CD-quality (low-noise metalic casettes with Dolby type-s noise reduction is said to achieve a CD quality though), as there is some characteristic high frequency hiss noise in those commercial casettes, especially on the lower quality tapes. Therefore I will take 60 dB as an average estimate. Also note that this SNR depends on the frequency, as the noise increase by frequency, but I will ignore it too.