14 Frequency response

3GPP46.008Half rate speechPerformance Characterization of the GSM Half Rate speech codecRelease 17TS

The frequency response of the GSM half rate codec, has been evaluated by computing the logarithmic gain.

The codec has been tested in error free condition only, without DTX associated, by independently processing 198 sine waves files spaced by 20 Hz and spanning the range between 50 to 3 990 Hz. Each file had a duration of 8 seconds and the input signal level was fixed at -22 dB (Vmax = 2 603).

The gain of the codec has been calculated by means of the formula:

Figures 1 and 2 report the logarithmic gain for the whole range of tones considered and for telephone bandwidth respectively.

Both figures show that the codec provide a flat frequency response in the telephone bandwidth, with the algorithmic gain confined in the range ±0,2 dB with a very few outliers.

The highest attenuation observed is 0,65 dB and occurs at 1 150 Hz.

It shall be noted that small deviations from these figures can be observed by using different levels and/or different initial phases for the sinewave signals.

Figure 1: Frequency response for the whole bandwidth considered

Figure 2: Frequency response in the telephone bandwidth