Difference Between an 8 Track and a Cassette Player

This is a follow on from my last article called the Next Generation of Cassette Decks, talking about improvements made with version 2 prototype. With this version I wanted to be able to test both 8 track and cassette tapes from a common development board, as I could not find a reel to reel tape machine at the right price.

The key differences between these formats are:

• The tape speed of cassettes are at 4.75 cm/second and 8 track are at 9.53 cm/second.
• Number of track, 4 for cassette and 8 for the 8 track as in it's name.
• Tape width of cassette is 3.81mm and for 8 track is 6.35mm.
• Cassette tape has two tape spools where 8 track has one (an endless loop).

Improvements were made with the low noise amplifier (LNA) design, alongside better isolation between the digital and analog circuits. As well as more simplified circuit layout made it possible to test a range of possible magnetic bias options.

Here is the block diagram of the working prototype, as a basic over view of both playback and record parts. As the signal level that comes in from the playback heads is at a very low level, it is important to minimize any unwanted noise at the low noise amplification (LNA) stage. Equalization (EQ) is must as the frequency response from the play heads and the tape need to be flattened out and with a low-pass filter to minimize the noise. The next stage is the noise reduction block, that is common to both record and play parts to provide an extra level noise rejection. As it is important when using ferric oxide tape, as it make overall improvement with keeping noise levels low and to make best out of this tape formulation.

The big difference is how the record section works, where there are both AC and digital bias circuits used, making it possible to compare the performance between them. The field programmable gate array (FPGA) provides an AC waveform to analog bias and the erase heads, from common clock for the micro-controller and the FPGA. The analog interface uses two phase pulse converter to sample the left and right audio inputs, where the FPGA converts this to a digital bias signal, based on my analog digital modulator (ADM), as used with class Q amplification. Another improvement is how the high voltage bias power supply is driven from the FPGA, providing for a wider operating voltage range.

The printed circuit board layout was done in a way to keep the digital and analog sections isolated, to minimize unwanted noise from the digital side. Only the top part of the board is required for the playback amplification, where the record processing uses the largest space, leaving what is left for the micro-controller. Another issue has been getting the integrated circuits, as there is not a single chip made for this recording technology, as it took months of waiting to get all the parts together with the current chip shortage.

With both 8 track and cassettes having very different record, erase and playback architecture, this is where this board layout came into it's own, providing the flexibility for these two formats. Record equalization is just important as playback if not more so, as magnetic tape does not have a flat frequency response, so therefore this needs to be EQ out. This is what sets out a high-end tape deck from a standard ones, all comes down the EQ design used, this is why you only see low end cassettes recorders these days. With this version two design it would be a mid-range cassettes deck. As for 8 track performance this is well a head of all the other decks I have tested so far, I put this down to the end of developments in the 1980's for this format. Another advantage is been able to use C type noise reduction, that was not about in the 1970's as tape hiss is a major issue with the 8 track format, far more so than with cassettes.

Limitations again are the magnetic formulations used, as we should of stop using and making ferric oxide tapes in the 1990's, as it is not possible do much with higher audio frequencies, with tape speeds below 19cm/second. With ferric cobalt formulations recording and playback of the higher audio frequencies above 10kHz is possible, making the sound quality a lot closer to that of compact discs.

Advantages of digital bias are better higher audio frequency performance in both cassettes and 8 tracks, using ferric oxide tape, with ferric cobalt the difference is not as noticeable compared to AC bias, note, I was not able to test this with 8 tracks as I could not find a ferric cobalt cartridge to experiment with. In the case of 8 track cartridges, there is very fine balancing point between recording audio above 10kHz and going into magnetic saturation at these frequencies. Therefore on the next prototype I will need to do a lot more with the EQ circuits to add a record and playback trim, to let the user to set levels above 10kHz to minimize this amount of magnetic saturation taking place. As for disadvantages with digital bias, is the digital noise that is generated within digital circuitry, as this is common to any high-end audio equipment that has a digital interface.

Future plans, I am thinking is making parts of technology open source making it possible for manufactures of magnetic recording systems to make improvements, and hopefully less junk been made. Another idea is making a limited number of 8 track player / records, as this is where I got the best all round performance, but getting the parts to do so, is not going to be easy. I also would like to get back to doing more with the tape tracking technology and the adaptive noise reduction, as my focus up to now has been on improving the overall sound quality.

Difference Between an 8 Track and a Cassette Player

Source: https://www.linkedin.com/pulse/8-track-vs-cassette-tape-grant-taylor

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