Thomas Edison invented a recording device in 1877. His invention recorded sounds on a tin foil cylinder. The device worked completely mechanically, with the sound vibrating a diaphragm that was connected to a needle that scratched the foil. A second needle could play back the recorded sounds through a large horn that amplified the sounds to listening levels.
This device did not produce great sound but it did provide recognizable sounds, or even music. It also provides for a great introduction to sound recording. You could even build a replica of the original Edison recorder by following these instructions. It looks like fun and would be a great conversation piece for the music fan.
Building on the techniques employed in the Edison recorder eventually led to phonograph records that stored the vibrations mechanically in the groove of a vinyl disk. The advantage of vinyl is that it is fairly durable and inexpensive to produce in quantity.
During the development stages of the phonograph record, it was also discovered that the sounds could be magnetically recorded on wire, and eventually tape. This was more convenient during the recording phase, but until the creation of tape cartridges (8-track and then cassette), it was not too convenient for listening.
In either case, the recordings are analog recordings. The vibrations are now electronically amplified and sent to speakers. Speakers in their simplest form are merely diaphragms. The diaphragm is moved magnetically.
If we were to route the signal to an oscilloscope (an instrument for viewing electrical waveforms) instead of speakers, we would see the waveform of the sound. The waveforms for sound, especially music, can be quite complex.
Fortunately, the oscilloscope lets us slow the signal down so that we can see just a small portion of the sound to show it better. In the diagram below, you can see that the waveform flows smoothly.
In 1982, Sony and Philips released the Compact Disc (CD) as a replacement for vinyl record albums. By 1988, the sales of CDs exceeded record sales. CDs use a digital approximation of the original music. The idea was to come up with a convenient and yet durable medium. We use the same technique for WAV files and other digitally recorded files.
To get a digital recording, we utilize an analog-to-digital (A2D) converter. It gives us a digital value for any given analog value. We take a series of samples at some given frequency and store each successive value. In the diagram below, the green bars represent separate digital samples.
If we take those digital values and send them through a digital to analog converter (DAC), we will get a waveform similar to the original waveform. But as you can see in the diagram below, it is not an exact match.
It is probably close enough for you to recognize the sound but it is not a very high quality. Part of the problem is the resolution, or precision, of our analog-to-digital converter. The first example uses a three-bit sample with eight gradations. We get a little closer if we increase the conversion to a higher resolution sample. But it is still not close enough.
The reason that our example does not match is because the analog signal changes faster than the sampling rate. If we increase the sample rate and resolution, we can get very close.
In the case of CD sound, we take 44,100 samples per second and use a 16-bit A2D. This gives us 65,536 gradations to the samples, and a sound that most people can not distinguish from the original analog signal. As you can see from the following diagram, by sampling faster and with more resolution, we get really close to absolute fidelity.
If we play the samples back for another comparison, you see that the signals are almost identical. Most people will not be able to hear the difference. I know I can not.
But some people can and this has resulted in the creation of a new audio format, DVD-Audio, that increases both the sample rate and the resolution. It should satisfy even the most sensitive ears. It will take up to 192,000 samples per second with a resolution of up to 24 bits. The frequency response will be 0-96 kHz instead of the 5-20 kHz a CD provides. It will also provide a dynamic range of 144db instead of 96db, combined with the ability of the format to handle the 6 channels (5.1 compatible) of sound instead of stereo.
This specification includes space for additional information. Each track can have up to 16 graphics still images that can be used for lyrics, pictures, and liner notes.
I have seen ads for the new DVD-Audio players in magazines so they should start appearing in stores soon. The first batch of DVD-Audio discs is due in early November. I note with interest that Brain Salad Surgery by Emerson, Lake & Palmer is on the November 7 list. I do not expect to buy my CD collection in this new format, but I have not heard the format yet.
