4.2. Features of speech signal conversion in the GSM standard

For the analog-digital conversion of speech signals in the GSM standard, ADCs are used.

Let us briefly review the main characteristics of speech signals (as recommended by CCITT) in the GSM standard and the characteristics of their analog-to-digital conversion:
- the frequency range of the speech signal is limited: 300 ... 3400 Hz;
- the duration of the sounds of a speech signal is from several tens to several hundred milliseconds, with an average value of 130 ms;
- for vowel sounds, the average duration is 210 ms, for consonants - 92 ms;
- the power spectrum of the speech signal has a maximum near the frequency of 400 Hz and falls at higher frequencies with a speed of about 9 dB per octave;
- during a telephone conversation, the level of the speech signal varies in the range of 35 ... 40 dB, while the level of consonants is on average 20 dB below the level of vowels;
- in analog-digital conversion and digital processing of a speech signal, they are limited to the frequency interval of a conventional analog telephone 300 ... 3400 Hz, while the coding of a speech signal takes into account a quasi-stationary Gaussian process, in which the spectral-correlation characteristics are constant over the interval
20 ... 30 ms

Additionally, the following should be noted:
- the auditory sensation of loudness is approximately proportional to the logarithm of intensity (~ In /, where I is the intensity of the sound);
- the threshold for hearing sound level does not exceed 1 dB;
- the human ear is weakly sensitive to the accuracy of the transmission of the phase relationships of the spectral components of the signal;
- the time constant of hearing on average is: with an increase in the signal - 20 ... 30 ms; during a recession - 100 ... 200ms.
Returning to the process of converting an analog speech signal into digital form, we consider in more detail the features of the sampling and quantization processes in the GSM standard.

Speech Sampling
Two signals are sent to two inputs of the sampler:
- U \ (t) - analog signal (speech);
- UJJ) - signal from the clock generator.
According to the discretization theorem (Kotelnikov-Shannon), if for the function U \ (t) the spectral component of the highest frequency is / тах (in the case of the speech signal / тах = 3400 Hz), then the instantaneous samples taken with the frequency fa> 2 / max , contain almost all the information of the original message (that is, fd> 2 * 3400 = 6800 Hz).
As is known, the application of the discretization theorem for telephony, where the speech signal (or facsimile signal) is limited by the frequency / тах = 3400 Hz, the sampling frequency is chosen equal to fd = 8000 Hz, that is, the sampling period is Td = 1 tfd = 125 μs.
Thus, at the output of the sampler, as a signal multiplier: U \ (f) U ^ t) = U2 (t \ the C / 2 signal is obtained (0 *

Quantization of the U2 (t) signal
In standard ADCs used in digital telephony, usually the number of quantization levels (at an acceptable signal-to-noise ratio) is set to 256 = 28, i.e. each digital signal level will correspond to a digital signal £ / 3 (at the appropriate sampling time) in the form of 8- bit binary code.

The GSM standard uses 8 bits of digital information per quantized sample. The frequency of the quantization signal generator Uk must correspond to: 8kHz-8 = 64 kHz, and the period 7 * = 1 / 64-103 = 15.625 μs, that is, the standard for the information transfer rate at the ADC output — 64 kbps — is used over one telephone channel.

It should be noted that 16-bit Sigma-Delta ADCs are used in GSM mobile communication systems, with a digital output rate of 128 kbps.

Consider the features of the digital-analog conversion of speech signals in the GSM standard.
As is known, the conversion of a digital stream carrying information about speech signals and speech coming from a decoder is implemented by digital-to-analog converters (D / A converters).

Modern DACs in mobile stations are made in the form of integrated circuits. Widely used are 16/18/20/22/24-bit DACs on a single integrated circuit. Cellular mobile communication systems use such a DAC, in which the input digital stream has a speed of 128 kbps. Such DACs convert digital streams into speech signals, which in the transmitting part were converted to digital signals into ADCs with a 16-bit sigma-delta conversion. In more detail the features of the circuit design and the principles of operation of the ADC and DAC in mobile communication systems can be found in the literature [4.9, 4.11].