Bournemouth — May 17th 2016
John began by introducing the basic principle of sound as a pressure wave which is captured using a microphone to produce an electrical analogue of the pressure wave. Up until the 1980s, this electrical signal remained in analogue form as it was recorded (e.g. onto tape) and later played back through an amplifier and speakers. This process worked well but has limitations since each step in the recording and playback process added some additional noise and distortion to the signal, giving rise to such problems as hiss, wow, flutter etc.
A system of digital encoding of sound waves was proposed by Alec Reeves in 1937, called Pulse Code Modulation (PCM). John explained how this involved sampling the wave at intervals and expressing each sample as a binary number. The number of samples per second and the number of bits used to encode each sample determine the quality. John then demonstrated recordings made at different bit rates from 'telephone quality' up to the standard adopted by Sony and Phillips for the audio CD which came on the market in 1983. These examples were played over Airplay through a pair of Sonos 5 speakers provided by Solutions and it was agreed that the CD quality recording was discernibly better.
The problem was that one minute of audio recorded in CD quality requires about 10.5MB of storage, so a more memory-efficient system was needed. This came from the Motion Picture Expert Group (MPEG) who, in 1993, were looking at how to get 74 minutes of video onto a standard CD. As part of their work, they included three definitions of how the audio part could be processed, the third definition being known as MP3. This system uses perceptual encoding which takes advantage of characteristics of human hearing to encode the sound more efficiently. More information on how MP3 works can be found at http://www.soundonsound.com/sos/may00/articles/mp3.htm
John then demonstrated CD quality vs. three levels of MP3 quality. MP3 at the highest rate of 320kb/s began to rival CD quality and split the audience somewhat. John made the point that the acceptability of the quality depends on the listening environment. Even 64kb/s MP3 is useable in a car. The sample can be found at https://www.dropbox.com/s/4wbsw5lk7oxovls/WAMUG_Music_Files.zip?dl=0
After a short coffee break a comparative listening test was carried out between the pair of Sonos 5 speakers and a B&O 'Zeppelin' with samples from the catalogues of Beatles and Beethoven. The Beethoven resulted in a clear preference for the Sonos 5s.
The usual Q&A session was then held, addressing problems with two factor verification and a question on streaming and buffering in video calls.
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