Making sounds with analogue electronics - Part 9: Sampling in an analogue environment

[Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses voltage control - "one of the major innovations in the development of the synthesizer." Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters. Part 3 discusses envelopes - the overall 'shape' of the volume of a sound, plotted against time. Part 4 looks at amplifiers as well as other modifiers, including LFOs, envelope followers, waveshapers, and modulation. Part 5 shows how a subtractive analogue synthesizer can be a learning tool for exploring some of the principles of audio and acoustics. Part 6 considers other methods of analogue synthesis. Part 7 deals with the topology of the modules that make up a typical synthesizer and then looks at categorizing types of synthesizers. Part 8 looks at how the basic synthesizer technology has changed from valves and transistors towards microprocessors and custom ICs.]

Environment
For brevity, this section will use the phrase 'analogue synthesizers' to mean analogue synthesizers of the monophonic, polyphonic and modular varieties, as well as string synthesizers, electronic pianos, bass pedal synthesizers and other analogue electronic musical instruments.

3.8 Sampling in an analogue environment

3.8.1 Tape-based
Audio recording and playback (in this context: 'sampling') based on tape recording techniques has a long history. The first 'tape' recorders did not use tape at all, but used wire instead. Plastic tape covered with a thin layer of iron oxide is much easier and safer to handle than reels of wire, and far easier to cut and splice!

Tape recording
The underlying idea behind how a tape recorder works is very simple. The sound signal is converted into an electrical signal in a microphone, and this signal is then amplified, converted into a changing magnetic field and stored onto tape. By passing this magnetized tape past a replay head, the changes in the magnetic field are picked up, amplified and converted back into sound again.

Magnetic tape is made up from two parts:

1. A plastic material which is chosen for its strength, wear resistance and temperature stability.
2. Magnetic coating which is chosen for its magnetic properties.

It is actually possible to record and replay sounds using a fine layer of iron oxide dust placed onto the sticky side of an adhesive tape, although this is not recommended as a practical demonstration. The commercial versions of recording tape are just more sophisticated versions of this 'rust on tape' idea.

A tape recorder is a mixture of mechanical and electronic engineering. The mechanical system has to handle long lengths of fragile tape, pulling it across the record and replay heads at a constant speed, and ensuring that the tape is then wound onto the spool neatly. This requires a complex mixture of motors, clutches and brakes to achieve.

The pulling of the tape across the heads is achieved by pressing the tape against a small rotating rod called the capstan. The tape is held onto the capstan with a rubber wheel called the pinch roller. The spool that is supplying the tape is arranged so that it provides enough friction to provide suffi cient tension in the tape to press it against the record and replay heads as it is pulled past.

Once past the capstan and roller, the tape is then wound onto the other tape spool. When the tape is wound forwards or backwards, the pinch roller is moved away so that the tape no longer presses against the capstan or the heads, and the spools can then be moved at speed (Figure 3.8.1).

The electronic part of a tape recorder has two sections: record and replay. The record part amplifies the incoming audio signal and then drives the record head with the amplified signal plus a high-frequency 'bias' signal. The combination of the two signals allows the response of the magnetic tape to be 'linearized'. Without the bias, the tape recorder would produce large amounts of distortion. The replay section merely amplifies the signal from the replay head (no bias is required for replay).