The first noisy suppressor circuits appeared in the market around early seventies. Among them the most used ones were sold under the trade names: DBX and Dolby
Therefore, even considering the improvement brought by such innovations, the recording sound analogically still presented distortions.
Such technical situation was based in the new measuring parameters used for the evaluation of the recording dynamic range allowed by the three known recording media: magnetic tape, discs and frequency modulation broadcasting mode.
Thus the dynamic range or the quantity of signal related with the noisy level and the distortion were far behind the wanted quality since the recorded sound still had spurious signals. Fig 334


Music and noisy: music consists of sounds of different pitch and loudness, separted by intervals of silence shown in the diagram by decrescnt lines.
Noisy or hiss is due to the physical-chemical of the midia; they are recorded with the sound signals and so reducing the music level.

The Dolby B system operates in the following way:
a) Before the recording it listens to the music to find the places where the nois might later be heard in the playback.

b) During the recording, by using an electronic method, it increase automtically the music volume in such way it is recorded at a higher level thant the normal.

c) During the playback, the volume is automatically reduced in all the places where it was increased during the recording.

This makes the music sound exactly right, because the loudness of every note is just the same as it was at the start at the same time the noisy of the tape, which is now mixed with the music, is reduced in all of the same places.
Fig. 334 - The principle of the Dolby noisy reduction system, invented by the American electronic engineer Dr. Ray Dolby used in the majority of domestic tape recorders.

By a careful evaluation in each stage of the audio chain, the engineers found that to improve the quality of the recorded sound it was mandatory to concentrate the researching efforts in the recording area in such way to be able to develop a new higher dynamic range and distortion free sound recording technique.
Around 1980 the engineers started to use the “pulse code modulation” in order the analogical signals from the original sound source could be digitized either for recording and playback. In the reality the pulse code modulation technique, as known as PCM can be defined as a modulation method requiring code to digitize a analogical wave.
Basically its principle consists that the original musical signal is sampled at a fixed frequency, and the value of the sampled level is ascertained. Fig 335

Step where the analog signals are sampled in thousands of parts in such away the same could be recovered entirely again.
Step where the signal sampled is then quantized. During quantization, a continuous amplitude analog signal is processed to produce a signal consisting of a finite number of discrete levels of amplitude and so it can have a slight discrepancy or the quantization error as know as quantization noisy. The error can be reduced in such away for every increase in number of steps used, there is a coresponding reduction in height of each steps.
Step where the quantized sample values are converted in a binary code.
A reverse step where the coding values is converted in quantized samples, which have same values as before the quantization.
Low-pass filter
It is responsible for the original signal recovery considering only the sampling parts contained in the original signal and so cuting off the entire undesired frequency spectrum generated during the sampling.
Fig 335 - The diagram of the PCM technique.

The value of each samples level is then converted into binary notation. Fig 336
Thus, for example, the resolution of the system used could be either 14 or 16 bits. The configuration of bits used is known as the quantization bits, and it is accepted that 14 or 16 bits systems are mostly common used. The PCM modulation method is complete when digital values representing each original sampled level are collated.
The PCM signal encoded in this way has the following major advantages: it is immune to distortion. Even if the original signal is subject to distortion, sampling at a fixed level allows the reproduction of the original in as near perfect way a way as possible.

Decimal Binary
1 1
2 10
3 11
4 100
5 101
6 110
7 111
8 1000
9 1001
10 1010
11 1011
12 1100
It is based on ten different numeral from 0 to 9. Numbers great than 9 are indicated by combinations of the ten basic digits in different columns as the “undreds”, “tens” and “units”.
For instance: 10
The combinations of use the two different numerals only or 1 and 0. To express numbers greater than 1 it must use other columns such as “twos”, “fours”, “eiths” and so forth, similar to the decimal system.
For instance: 1010
Practically speaking the binary digits indicate a permanent yes and no mode. In the PCM system uses pulses to express the sampled values or they can only express” on” or “off”. By using a decimal system it cannot use ten different numerals to express these patterns of the PCM pulses.
Fig. 336 - Chart comparing the decimal and binary notation.

Multiplexing or many different signals can be mounted in one PCM signal without any crosstalk between the different signals.
The signal-to-noisy ratio, which defines the musical quality, is related with the number of quantization bits used at the time of reproduction. Thus, if a large number of quantization bits are used, it is possible to obtain an astoundingly signal-to-noisy ratio far beyond in the analogical method.

Fig. 337 - The videodisc launched in the market by Philips circa 1970. Fig. 338 - The Compact audio disc
Courtesy: Philips Brazil, 1980.

The PCM technique for audio application started to be used in later sixties in Japan, by using helical scans video tape recorders. Considering the bandwidth needed to record a video signal on videodiscs was more than the needed to record a digitized sound signal, the same system could be used for the former purpose.
Bearing those aspects in mind several types of equipments were developed in order to improve the recording density, as while in the analogical recorders require an ac bias to achieve the necessary linearity in the PCM tape recorders do not. Thus, engineers developed the either the rotary head video tape recorders as well as the stationary head types, capable of recording and dispersing one channel of data on a plurality of tracks.
In 1980 such kind of PCM recorder was used to record high quality “Long Playing” discs.
Therefore, soon new types of equipments and recording techniques were developed as the DAD or Digital Audio System – fig 337
In mid eighties the companies Sony and Philips merged their digital techniques acquired from the former DAD method and so they suggested the well-known and universal adopted Compact Disc as known as CD. Fig 338
This new method to record the live sound has revolutionized the phonographic industry. It was a direct consequence of the merging LASER technology with the digital reproduction, which certainly improved either the quality of the recording as its popularization by using a practical compact and full automatic disc player.