MP3 player loading troubles on ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: MP3 player loading troubles on ZX81

Post by 1024MAK »

With a normal audio signal, the positive going part (half cycle) should be the same relative level as the negative going part. This does assume that the frequency / period of both parts is the same.

The polarity from a cassette tape deck can be either way round, as human ears can’t tell the difference with sound signals. So it makes little difference if the loudspeaker diaphragm moves forward and then back, or the other way round (unless you have stereo and wire your two loud speakers up opposite/differently).

Inversion of the signal can occur in any of the circuitry from the coil of the magnetic tape head, the preamp, power amp, or in the microphone input circuitry.

And who knows which polarity and how many signal inversions occurred in the duplication equipment for ‘prerecorded’ software tapes?

Hence computer tape systems should be designed to cope with either polarity. The ROM loader therefore should cope with either polarity of signal. But this does not mean that all custom loaders will be okay.

The ULA in the ZX81 completely ignores the negative part of the waveform and only looks for the positive part going above a certain value. This value is determined by what voltage the transistor in the ULA switches on. The ULA and hence the transistors in it, being intended for use only with positive digital signals. Hence they ignore any low or negative voltages.

Because the ULA tape input pin uses fairly simple circuitry, it can be affected by any electrical noise, either from the recording on the tape, the audio equipment, poor or dirty connectors, or noise from the computers own circuitry. The circuitry is especially sensitive if the signal is near the switching threshold, as the noise spike can either temporarily increase or decrease the voltage just past the threshold voltage, hence causing a distorted digital representation of the input signal (just like a jolt or bump in the road can wreak your handwriting if you are writing something while travelling in a car, bus, coach…). Sinclair made some improvements to the ZX Spectrum ULA to reduce this problem and hence improve the tape loading on that machine.

If you look at the ear circuitry for the ZX80, the audio signal goes to series resistor R29 (180Ω), then coupling capacitor C12 (47nF). Then resistor R1 (1kΩ) acts as a load. The signal then goes into chip IC10 (74LS365).

The ZX81 ear circuitry is similar, but the values are different, plus the load resistor is first rather than later in the circuit.

So first there is the load resistor, R34 (220Ω), then coupling capacitor C10 (10nF), then series resistor R33 (4.7kΩ), and finally the ULA input pin (20).

And the ear circuitry for the ZX Spectrum is similar as well.

The purpose of the coupling capacitors is to stop any DC voltage from passing through (in either direction). They only allow (or couple) ac signals to pass through.

The purpose of the series resistors, is to limit the current flowing into or out of the digital chips. Hence reducing the likelihood of damage if the volume is far too loud.

The purpose of the load resistors, is to provide a minimum load for the amplifiers in the cassette deck so that they work correctly. Plus they provide a discharge path for the capacitors.

Mark
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XavSnap
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Re: MP3 player loading troubles on ZX81

Post by XavSnap »

Many thanks Mark !

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Xavier ...on the Facebook groupe : "Zx81 France"(fr)
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