11 Performance with DTMF tones
3GPP46.008Half rate speechPerformance Characterization of the GSM Half Rate speech codecRelease 17TS
11.1 Introduction
In the fixed telephone system, DTMF (Dual Tone Multi Frequency) signals are transmitted in the speech channel for signalling. This has led to the use of DTMF tones for applications such as the control of answering machines and mail/messaging boxes. In the GSM system, the handling of these signalling tones is dependant on the direction the signal is travelling. If it is in the uplink (from the mobile station to the network), the signalling channel is used, rather than the speech channel. In the downlink (from the network to the mobile station), these tones are carried in the speech channel. Even though it was not a requirement for the half rate speech channel to be able to carry these tones in the downlink, their transmission was tested.
11.2 Test set-up
16 DTMF signals are defined representing the 10 numeric keys, the characters "A", "B", "C", "D", "*" and "#". Each digit consists of two sine signals of distinctive frequencies, one chosen out of 4 values from the low frequency group (or row frequency), and one out of the 4 values from the high frequency group (column frequency). Both frequencies are sent simultaneously ideally with same amplitude and at exact frequency values. For practical use, certain tolerances of the frequencies and of the signal amplitudes are specified.
A DTMF receiver must be capable of detecting these tones. It should detect all the DTMF tones even under noisy conditions or when speech is present. Also, it should not interpret other signals from the voice band as valid tones. The tones can only be distinguished by their specific frequency and amplitude composition so it is important, if they are to be recognized by the half rate system, that they conform to the CEPT recommendation T/CS 46-02 (1985). Among others, the difference in the amplitudes of the 2 components (twist) shall not exceed 6 dB. The minimum signal length from sending unit is 75 ms while a 40 ms signal should be detected at the receiver side. Pauses from the generator shall last 65 ms while the receiver shall detect 20 ms.
The DTMF tests were done at nominal frequencies with different pulse and pause duration and different amplitude levels on a PC based set-up. DTMF signal files were generated by means of a DTMF software package for the 16 signals with 10 samples for each tone. After processing with the HR-codec software, the result files were input to a DSP based hardware with a standard DTMF recognition S/W meeting CEPT requirements. All experiments were done also with modified DTMF receiver software. The tables in subclause 11.5 list the number of recognized tones.
All dB values mentioned are for each individual component of the DTMF signal, with reference to the overload point.
11.3 Results
The results of the test with a standard DTMF detector are shown in table 7. Even at ideal conditions with nominal DTMF signal frequencies, no additional signals in the speech band, and error free transmission, the recognition is poor after processing. Only with a relatively high level of -12 dB and a tone length of 80 ms is a 100 % recognition achieved. Under all other conditions at least one tone shows severe problems. There is no linearity in this experiment, e.g. "4" is recognized well at -18 dB level but very poor at -22 dB while "7" shows the opposite behaviour. Also, when the twist is reversed, the results differ in ways which depend on the code being transmitted. The recognition of very short tones (40 ms) is not acceptable, and the longer tones (120 ms) are problematical too.
A reason for the poor behaviour might be a time dependent twist generated by the GSM Half Rate codec when one of the two components develops differently from the other due to the non-linear behaviour of the codec. For more than 40 ms the twist at certain DTMF tones was observed to be greater than 6 dB and thus out of the allowed range of the specification of standard DTMF receivers. In experiment (h) with -12/-18 dB signals and 120 ms tones, 10 inputs of "A" resulted in 12 recognitions. A slow oscillation of the signal amplitudes may have generated a twist of more than 6 dB for longer than 20 ms. This made the detector observe a valid pause and a new tone, increasing the number of detected tones above the number of input tones. This might have happened also for other tones under the condition (h) where e.g. 10 detected tones may result of 8 correct detections, a double detection from one input and one failure. The test equipment could not decide such effects – as also in practise just the result counts. At 80 ms twist signals, such slow oscillations do not have the same effect because under no condition a valid 2nd tone can be detected (40+20+40>80).
Table 8 shows the results of the same experiments as described above with a DTMF detector tuned for recognition in GSM half rate speech codec transmission. Using knowledge of the possible reasons for detection errors in the tuned detector, the detection rate was improved. However, even at the still ideal signal conditions as described above, the results were not satisfactory where there was severe twist or short (40 ms) tones. Also, the modifications may well increase the acceptance of non-DTMF signals as valid DTMF tones. This, however, was not tested.
11.4 Conclusions
With the standard detector the recognition rate averaged over all experiments was 74 %. The tuning of the detector for the half rate channel characteristics could improve the detection rate to 92 %. As all experiments still had rather ideal conditions, in real application an even lower rate for detection has to be assumed, also due to the misinterpretation of other signals in the modified detector.
In conclusion, a serious commercial application using DTMF in the speech channel should not be supported with the GSM half rate codec.
11.5 Result tables of experiments with standard and modified DTMF detectors
The tables below list the numbers of detected tones from 10 input signals at each tested condition. For twist conditions, the pair of level figures indicate the level of row frequencies and column frequencies respectively.
Table 7: Summary of DTMF tests with standard DTMF detector
|
Condition\Tone |
1 |
2 |
3 |
A |
4 |
5 |
6 |
B |
7 |
8 |
9 |
C |
* |
0 |
# |
D |
Total |
|
(a) -12 dB, 40 ms |
5 |
2 |
9 |
10 |
0 |
8 |
1 |
5 |
10 |
5 |
3 |
0 |
0 |
10 |
10 |
9 |
87 |
|
(b) -12 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
160 |
|
(c) -18 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
8 |
10 |
1 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
149 |
|
(d) -22 dB, 80 ms |
10 |
10 |
10 |
10 |
4 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
154 |
|
(e) 3 dB twist, -12/-15 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
7 |
10 |
10 |
10 |
0 |
10 |
10 |
10 |
147 |
|
(f) 6 dB twist, -12/-18 dB, 80 ms |
0 |
1 |
0 |
10 |
0 |
10 |
0 |
10 |
0 |
10 |
0 |
4 |
0 |
0 |
10 |
0 |
55 |
|
(g) 6 dB tw. reverse, -18/-12 dB, 80 ms |
10 |
10 |
4 |
10 |
6 |
3 |
0 |
0 |
0 |
0 |
10 |
10 |
0 |
0 |
6 |
10 |
79 |
|
(h) 6 dB twist, -12/-18 dB, 120 ms |
8 |
3 |
9 |
12 note |
7 |
10 |
5 |
10 |
0 |
10 |
7 |
9 |
3 |
10 |
10 |
9 |
122 |
|
Total |
63 |
56 |
62 |
82 |
47 |
71 |
44 |
65 |
38 |
65 |
60 |
63 |
33 |
60 |
76 |
68 |
953 |
NOTE: 10 input signals in this test case resulted in 12 recognized tones. An explanation is given in subclause 11.3.
Table 8: Summary of DTMF tests with modified DTMF detector
|
Condition\Tone |
1 |
2 |
3 |
A |
4 |
5 |
6 |
B |
7 |
8 |
9 |
C |
* |
0 |
# |
D |
Total |
|
(a) -12 dB, 40 ms |
9 |
10 |
8 |
1 |
0 |
8 |
4 |
10 |
10 |
10 |
8 |
9 |
8 |
10 |
6 |
8 |
119 |
|
(b) -12 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
160 |
|
(c) -18 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
160 |
|
(d) -22 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
160 |
|
(e) 3 dB twist, -12/-15 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
160 |
|
(f) 6 dB twist, -12/-18 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
6 |
10 |
0 |
10 |
6 |
10 |
0 |
10 |
10 |
0 |
122 |
|
(g) 6 dB tw. reverse, , -18/-12 dB, 80 ms |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
10 |
7 |
10 |
10 |
10 |
7 |
0 |
10 |
10 |
144 |
|
(h) 6 dB twist, -12/-18 dB, 120 ms |
10 |
8 |
10 |
10 |
10 |
10 |
8 |
10 |
8 |
10 |
10 |
10 |
8 |
10 |
10 |
10 |
152 |
|
Total |
79 |
78 |
78 |
71 |
70 |
78 |
68 |
80 |
65 |
80 |
74 |
79 |
63 |
70 |
76 |
68 |
1177 |