Delay

Delay, as an audio effect, has been realized through many different methods over the years.

Filters can create a electronic delay effect, but the delay is usually very short, shorter than we can perceive with our ears. Filter delay, as an effect, is normally confined to phase shift effects only. A "phase shifter" or "phaser" effect can be sonically similar to a "flanger" effect (see below). Both refer to very short delay effects. Phasers require a RC network with an electronically variable R. A single stage will only provide a subtle effect. For dramatic effects, several stages are placed in series, and the variable resistances must be electronically ganged together. The variable resistance component can be a FET, an optocoupler (LDR + LED), an OTA (CA3080, CA3094, LM13600/700), or silicon carbide varistors (obsolete, used in '50s/'60s Magnatone amps; modern zinc oxide varistors are more like switches and will not work as linear variable resistances).

Tape Delay

Magnetic tape allows audio to be stored indefinitely and then played back at the user's convenience. The physical space between the record and playback heads creates an inherent delay that can be manipulated to design a delay effect device. The Echoplex is the most direct application of this idea, where the playback head is actually manually moved along a fixed track. Other units used fixed head positions, but allowed for manual adjustment of the motor speed to change the travel time from record to playback head.

Flanging

Magnetic tape was also responsible for allowing a very fascinating effect nicknamed "flanging" in reference to the flange of a tape reel. If a piece of music was copied from one reel to another, and then both of those reels were sync'd up and mixed together, the tape operator could create interesting filtering effects as the two nearly identical sources drifted in and out of sync. The "drift" could be manually manipulated by placing a finger on the flange of one of the reel. In the 1970s, solid state delay technology emulated the "flanger" effect by introducing a small delay line, modulating it, usually with a slow oscillator, and mixing the two together.

"Through Zero Flanging"

One difference between using two pieces of previously recorded sources, as opposed to one live source and one delay source, is that the two recorded sources can both be slowed down independently. How or why does that matter? If reel A and B are identical pieces of music being played back in sync, and reel B is slowed just a bit by "flanging," we'll hear this distinct filtering effect that any "flanger" effect can achieve. But, with 2 previously recorded sources (no live input) we can now leave B alone, and slow A until A actually lags behind B in the source material. This is nicknamed "through zero flanging" and it actually has a different and more complex "whoosh" effect that sounds as though the flange has gone "through" something instead of the "back and forth" or "round and round" effect that the regular flange effect has. Solid state and digital delays can reproduce this effect, but it does require 2 delay lines with independent control over the delay times - features that most "flanger" effects lack.

Magnetic Disc Recording

As an alternative to tape, a rotating magnetic disk was used by the Binson company (others?) to which magnetic erase, record, and playback heads were pressed against in the same way tape recorders work.

1959 Tel-Ray patent on an "electrostatic recording" apparatus.

Electrostatic Recording (the Tel-Ray "oil can" systems)

US Patent 2,892,898, applied for in 1958, documents a short lived and interesting alternative to tape that survives in a whole series of delay systems made under the Tel-Ray and Morley brands, who also made OEM products for Fender, Gibson, and others.

Solid State Delay (Charge Transfer Devices)

See also:

BBD History

In 1968, Philips started to work on a solid state delay based upon many sample and hold circuits strung together in series. The idea was nicknamed the "bucket brigade delay." There were many difficulties that took years to iron out. Relatively cheap commercial ICs weren't released until about the mid '70s. The first notable chip that audio manufacturers took to was the MN3001 from Panasonic, released in November 1974.

In 1969, AT&T/Bell Labs were working on a similar idea called "charged coupled devices." This technology seemed superior to BBDs, but also languished for a few years before the technology was usable for audio effects.

Both BBDs and CCDs can be lumped together as "charge transfer devices," or CTDs. This terminology never seemed to be stringently adhered to, and both manufacturers and designers routinely seemed to mix up the terms. If properly designed, there should not be an apparent difference between a BBD echo effect and a CCD echo effect. Several notable models of guitar delay actually switched from BBD to CCD over the life of the model.

The Marshall Time Modulator is notable for not waiting for cheap commercial IC development, but using custom CCD ICs to achieve the best possible audio performance specs.

Sample and hold is a method of performing analog to digital conversion. BBDs essentially perform the first step of a analog to digital conversion over and over and over again for the sole purpose of delay and nothing more. BBD circuits are not digital because the analog value is never quantized into binary. However, BBD circuits do have clocks, they do sample, they are subject to the Nyquist theorem, they require anti-aliasing and reconstruction filters, so they do resemble digital audio circuits and can be confused as such.

BBD chips struggled to overcome problems with distortion and inherent low pass filtering due to the cumulative effect of all the capacitor stages. It is not uncommon to find BBD chips that have distortion problems.The NOS chips appear to be of better quality in this regard than many of the Chinese reissues. Noise is a particular problem with BBD chips. Low noise chips have to be hand selected. It is very common to include a compander IC (almost always the NE570/571) to aid in noise reduction as this could avoid having to select low noise chips.

Digital Delay

As digital technology matured in the 1970s, digital audio reached a point where it could be used a a storage medium that could rival tape technology. The digital implementation of delay is usually analogous to the tape delay system. Audio is converted to digital representation, saved or "written" to a memory block, then after a set delay time the information is "read" and converted back into an analog audio output. Early digital delays computers can be made up of 1970s IC chips, where each function is carried out in hardware by ICs, writing to large rows of RAM chips. As IC technology progressed, the entire delay process was eventually boiled down into a single IC package which required an external memory chip or chips, and later the memory was also tucked into a single IC. The PT2399 is probably the most popular of the "all-in-one" digital delay ICs for the hobbyist. Commercial delay effects may use a custom IC or the insides may resemble a small computer with a microprocessor or microcontroller chip working with a dedicated DSP chip to achieve the best digital audio processing. Modern computers, tablets, cell phones, etc, have more than enough processing power to run whatever kind of delay you want. Delay is a standard effect you can access in most software that processes audio.