In order to enhance listening pleasure, many techniques have been developed over the years, some of the most effective relying on the brain's ability to process sound according to it's temporal contexts and content. Leslie speakers, by rotating a speaker in an upright organ or separate cabinet, would introduce phase differences or throw the sound to be reflected by differing surfaces thus offering an illusion of three dimensions. Many FM MPX decoders use output filters that allow mixing of frequencies above 2kHz to subjectively reduce noise without apparently affecting the stereo image.
When stereo FM networks were adopted in the UK the BBC chose a higher signal to noise ratio with a corresponding drop in separation, suitable for classical music, but the IBA chose higher separation and higher noise, more suited to 'pop'. Considerable interest was shown in circuits that could enhance stereo effects. The development of binaural and quadrophonic (SQ and CD4) systems met limited success, many put off by the cost of a decoder, extra amplifier and speakers. CD4 was considered to outperform lower spec SQ decoders, so a logic-enhanced version appeared. Binaural recordings did not use a decoder but relied on a specific listening room arrangement or headphones. For those on a budget, resistive mixers for speaker outputs appeared, one popular system based on the Hafler design.
Manufacturers would sometimes build in 'ambio' circuits with varying degrees of effectiveness. Below is shown a very simple circuit whereby two speakers were connected across the two power amplifier outputs via a coil.
With damping factors for the power amplifiers quoted as >12 in stereo and speaker switches of the same construction and rating as those used for the signal switching, limitations are obvious but were found favourable by many.
Another arrangement employed lower level mixing before the tone controls. This unit also offered the basic SQ decoder circuit (MC1312 IC) although more advanced logic-enhanced designs offering greater separation were available.
Some custom designs have included, as a complement to the mono switch, continuously variable width stages giving mono to super-stereo. A couple are shown below. The first, a simple but effective mixer module;
and a JLH version;
At a London Audio Engineering Society convention (1980?) periphonic sound was demonstrated by the NRDC-sponsored Ambisonic partnership. This originated out of work done in 1970 by Michael Gerzon at Oxford on tetrahedral recording. Eight speakers were each placed at the corners of two rectangles, one horizontal, the other vertical, making for a very unobtrusive arrangement. Decoder design had been simplified and a periphonic or soundfield microphone could offer mono, stereo, three-dimensional horizontal surround or periphonic options. Perhaps the requirement for multiple speakers and developments in digital techniques did not attract the market.
On occasion, extraction of a solo or other desired part from a stereo recording can be effected by an arrangement similar to that below.
A similar technique can be used to suppress a lead vocal, but the bass on one channel might have to be filtered so as not to cancel this.
Binaural recordings giving startling realism were made with two microphones situated in the ears of a dummy head. These retained the delays between each ear caused by the shape of the head and it's angle to the sound. As a consequence playback was normally via headphones to give out-of-head localization. The following cross-feed function circuit suits both headphones and loudspeakers and is recommended for experimentation with stereo recordings.
One of the most important sound-processing techniques to impart spatial qualites are delays. Reverberation, echo, space, vibrato, phasing, flanging, chorus, ensemble and string ensemble are just a few of the special effects that can be obtained by delaying an audio signal. The close-miking of instruments will often give a dead or flat quality as opposed to the fullness, body and ambience given when care is taken to include natural reverberation. Artificial reverberation can then be added, in a studio, to 'restore' these. Interesting reverberation / echo effects have been introduced using open-reel tape decks fed by room mics and mixer, the delays between record and play heads creating effective ambiences, some of subtlety, which were then retained on the tape. With judicious positioning of mics and speakers, the use of a tape-deck could be dispensed with. 'Echo chambers' were exactly that; an enclosure containing mics and speakers whose characteristics could be altered with curtains or tiles.
For long delays, where say a musician wished to 'accompany' themselves, two open-reel decks have been used in tandem. The instrument's output was recorded by the first deck the tape from which fell into a 'bin' (a large flat enclosure whose depth was the width of the tape) providing the delay. The tape then passed through the second machine (on to a take-up reel) the play output of which was then fed back into the mix. With care, the results in public performance can be very memorable. Nowadays however, since the advent of digital techniques, a plethora of rich manipulation is available.
In the late seventies, sophisticated analogue electronic delays became possible with the introduction of charge coupled devices (CCD) and bucket brigade devices (BBD) which offered distinct advantages over the mechanical limitations of the spring-line reverb unit and tape loops. A professional unit like the EMT250 used digital delay lines and MPU controlled RAM (128k) to give 19 different delay elements. Digital Delay Systems' Audio Pulse Model One used digital shift registers and delta modulator A-D and D-A converters to drive additional speakers on either side of the listener. For domestic use, WEGA's Acoustic Dimension Compiler (ADC-2) was available which fed two ancillary speakers in, say, a living room. The 'space' control varied the delay time, the 'reflection' control determined the degree of feedback around the delay lines and the 'characteristic' switch varied the high frequency response of the unit. A very impressive 'box of tricks' that contradicted the trend for a minimum of signal manipulation was the 1981 Carver C-4000 Sonic Holography Autocorrelation Preamplifier. This contained tone controls for each channel with variable turn-over frequencies, an auto-correlator and expander with variable threshold for each and the Hologram Generator with it's associated time-delay (20W) amps.
Adjusting the length and level of reverberation can tailor a hall for the specific music being performed. Longer delays are used for orchestral works, shorter ones for chamber music. The density of the audience will affect the outcome also, a 'fuller' house having a shorter reverb time than an empty one. A subjectively 'natural' reverberation will have a high density of reflected signals whose spacing will be non-periodic.
Several psycho-acoustic phenomena connected with the delaying of audio signals and how these effects can be exploited to subjectively improve room acoustics and studio recordings together with specialised areas of speech manipulation and pitch correction cannot really be done justice here and are suggested for independent study.
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