Signal Generator/Detector

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Commetrex’ Signal Generator/Detector (SGD) furnishes all necessary functionality at the physical- layer level to support in-band telephony-signaling functions. Specifically, the SGD implements signal- processing functionality to generate and detect DTMF and Multi-frequency(MF) (R1/R2) digits as well as any generic in-band tone comprised of up to two frequencies. The SGD is compliant with the MSP Consortium M.100 recommendation, the Texas Instruments XDAIS algorithm packaging standard, the ECTF S.100 requirements for signal generation and detection, and the ITU Q.24 recommendation.

The baseline SGD is implemented and maintained in the C-language. More resource-efficient instances of the product are available for specific processors, such as the Texas Instruments TMS320C5400 and ‘C6000 DSPs and the ARC International ARCtangent customizable processor. Commetrex also plans to use the SGD in its MSP PCM Interface Boards.

Signal Generator/DetectorDTMF signaling is a mechanism used in the PCM- TDM telephone network to represent digits with tone frequencies in order to transmit them over an analog telephone line. DTMF signaling is primarily used to encode the dialed number generated by a subscriber terminal that is to be processed by the subscriber’s access device. In addition, DTMF is popular in interactive applications, such as telephone banking or voice-mail systems, in which the user can select options from a menu by sending DTMF signals from the telephone.

The SGD generates DTMF mathematically using two digital oscillators. One of the most frequently used detection algorithms, and the basis of the SGD’s detection, is the Goertzel algorithm, which efficiently extracts the spectral information from the incoming signal.

Signaling Systems R1 and R2 are designed for information exchange over toll facilities. Signaling System R1 is used in North America while R2 is used in Europe. Both systems use MF tones for signaling.

Design Overview

The SGD can be thought of as a media-stream transform (MST). For receive, it accepts a PCM sample stream from the telephone network and transforms it to a data and event stream for an application program. For transmit, it accepts commands from the application program and generates a PCM stream.

For example, in DTMF/R1/R2 tone generation, the input stream is transformed (encoded) from a sequence of digits to combinations of specified frequencies. In DTMF/R1/R2 tone detection, the input stream is transformed (decoded) from a harmonic signal of specified frequencies to a sequence of digits. The signal (DTMF, MF/R1/R2, and a generic tone) generator is implemented in one MST, the DTMF and a precise tone detector in another MST, and the MF (R1&R2) tones detector are in a third MST. The figure below shows the above partitioning of the signal generator/detector MSTs.

DTMF/MF and Precise Tone Generation

The TONE MST accepts the following commands.

Input Commands

Command Name Description
Gen_dtmf Starts DTMF generation.
Gen _R1 Starts MF/R1 generation.
Gen _R2F Starts MF/R2F generation.
Gen _R2B Starts MF/R2B generation.
Gen _tone Starts generic tone generation.
Stop Stops the TONE MST

Parameters

The tables below show the TONE MST user-defined parameters for DTMF, MF, and a generic tone generation.

DTMF and MF Parameters

Parm Name Type Default Units Description
Level 1 INT -6 dBm The level of the low frequency component of the DTMF pair. Valid range – 43 to –3.
Level 2 INT -4 dBm The level of the high frequency component of the DTMF pair. Valid range – 43 to –3.
LongPause INT 5000 Msec The delay associated with the ‘.’ (period) character. Valid range 0-65535.
Off Time INT 55 Msec The duration of the silence time between each DTMF digit. Valid range is 0–65535.
On Time INT 45 Msec The duration of each DTMF digit. Valid range is 0–65535.
ShortPause INT 2000 Msec The delay associated with the ‘,’ (comma) character. Valid range 0-65535.

Generic Tone Parameters

Parameter Name Type Default Units Description
Level 1 INT -20 dBm The level of the first (or only) frequency component. Valid range – 43 to –3.
Level 2 INT -4 dBm The level of the second frequency component (if any). Valid range – 43 to –3.
OffTime INT 0 Msec The duration of silence between tones on time. Specify 0 for no off-time. Valid range 0-65535.
OnTime INT 200 Msec The duration of the tone. Valid range is 1–65535.
FREQ1_Parm_1 INT N/A N/A Parameter 1 of the first (or only) frequency of the generated tone. Must be S.10 fixed point format. This parameter is computed from the desired frequency.
FREQ1_Parm_2 INT N/A N/A Parameter 2 of the first (or only) frequency of the generated tone. Must be S.10 fixed point format. This parameter is computed from the desired frequency.
FREQ2_Parm_1 INT N/A N/A Parameter 1 of the second frequency of the generated tone. Must be S.10 fixed point format. This parameter is computed from the desired frequency.
FREQ2_Parm_2 INT N/A N/A Parameter 2 of the second frequency of the generated tone. Must be S.10 fixed point format. This parameter is computed from the desired frequency.
Iterations INT 1 Integer The number of times to repeat the alternating tone and silence period. A count of –1 means repeat forever*. Otherwise the valid range is 1 to 32767.

* Therefore, if the Iterations parameter is set –1 and the Off-time parameter is set to 0, a continuous tone is produced.

DTMF and Precise Tone Detection

The DTMF MST implements the DTMF tone detection and the generic precise-tone detector. The generic tone can be up to two frequencies.

Input Commands

The TONE MST accepts two commands: one for the DTMF detector and another for the generic precise- tone detector.The commands accepted by the DTMF MST are listed in the table below.

Tone Commands

Command Name Description
Detect_DTMF Starts detecting DTMF digits
Detect_Tone Starts detecting a specified generic tone
Stop Stops DTMF/Tone detection

Parameters

The tables below show the DTMF MST user-defined parameters for DTMF and a generic tone detection.

DTMF Tones Parameters

Parm Name Type Default Units Description
OffQualLevel INT -43 dBm The minimum signal level required to maintain recognition of a DTMF signal once recognition has started. The valid range is –43 to –15.
OffQualTime INT 40 ms The minimum duration of absence of a recognized DTMF signal before end-of-digit event will be emitted. The valid range is 5 to 32767.
OnQualLevel INT -39 dBm The minimum signal level recognized as a DTMF signal. The valid range is –43 to –15.
OnQualTime INT 40 ms The minimum duration of a recognized DTMF signal before a digit event will be emitted. The valid range is 23 to 32767.
MaxInterrupt INT 20 ms The maximum time which a digit may be interrupted before it is deemed ended. Usually range is between 10ms to 50 ms.

Tones Parameters

Parm Name Type Default Units Description
OffQualTime INT 500 ms The minimum duration of absence of detected tone before a tone-end event will be emitted. The valid range is 0 to 32767.
OnQualLevel INT -28 dBm The minimum signal level that will be detected. The valid range is –40 to 0.
OnQualTime INT 500 Ms The minimum duration of a detected tone before a tone event will be emitted. The valid range is 22 to 32767.
FREQ1_Parm_1 INT Parameter #1 of the first (or only) frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ1_Parm_2 INT Parameter #2 of the first (or only) frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ1_Parm_3 INT Parameter #3 of the first (or only) frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ1_Parm_4 INT Parameter #4 of the first (or only) frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ2_Parm_1 INT Parameter #1 of the 2nd frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ2_Parm_2 INT Parameter #2 of the 2nd frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ2_Parm_3 INT Parameter #3 of the 2nd frequency of the generated tone. This parameter is computed from the desired frequency.
FREQ2_Parm_4 INT Parameter #4 of the 2nd frequency of the generated tone. This parameter is computed from the desired frequency.
Bandwidth1 INT 70 Hz The bandwidth of the 1st frequency.
Bandwidth2 INT 70 Hz The bandwidth of the 2nd frequency.

The Input Data Stream

The input data stream consists of linear samples at 8000 sampling rate. Each call to the SGD to process the PCM stream is usually executed for 10 milliseconds of linear PCM data.

MF Detection

Commands

The MF MST accepts the following commands.

TONE MST Commands

Command Name Description
Detect_MF_R1 Starts detecting MF R1 tones
Detect_MF_R2B Starts detecting MF R2 backward tones
Detect_MF_R2F Starts detecting MF R2 forward tones
Stop Stops MF tone detection

The MF MST reports the following events:

Event Name (ID) Value Description
_BEGIN ‘0’-‘9’, ’B’-‘F’ MF digit begins.
_END ‘0’-‘9’, ’B’-‘F’ MF digit ends.
_DONE MF detection stopped.

MF Events

Tone Generation

The tone generation MST has a wait-for-dial-tone capability and the following performance specifications:

Frequency resolution +/-1 Hz
Dynamic level -33 dBm to 0 dBm (programmable), -9 dBm nominal
DTMF digits 0-9, *, #, and ABCD per ITU-T Q.23 and Q.24
DTMF Rate 10 digits/sec (programmable)
MF digits 0-9, KP, ST, STP, KP2/STP2, and ST3 per US(R1) and ITU-T Q.321 European MFC/R2 forward/ reverse tones per ITU-T Q.441
Generic tone type Single or dual
Duration 10 msec increments
Generic tone freq. Range 200 to 4000 Hz

DTMF Detection

Dynamic Range -35 to -3 dbm
Minimum tone duration As specified in Q.24; AT&T: 40 ms
Inter-digit timing Detects like digits with a 40 ms inter-digit delay. Detects different digits with 0 ms interdigit delay
Acceptable Twist As specified in Q.24; AT&T: +4 dB to –8 dB
Talkoff Passing the Mitel and Bellcore talkoff requirements
SNR 10 dB (reference to lowest amplitude tone)
Frequency Tolerance As specified in Q.24; AT&T: +/- 1.5% < BW < +/-3.5%

MF Signaling Detection

Dynamic Range -35 to –3 dBm
Acceptable Twist 6 dB
Talkoff TBD
Bandwidth +/2% Hz

Test Results

The Commetrex DTMF detector uses a windowing algorithm to meet all the frequency-tolerance requirements of Q.24. The following table contains the test results.

Decode Tests(All digits detected)
Frequency Tolerance Tests Frequency Pos Deviation Neg Deviation
697 2.00% -2.20%
Low Band 770 2.00% -2.00%
852 2.10% -2.00%
941 2.00% -2.30%
1209 2.50% -2.70%
High Band 1336 2.50% -2.70%
1477 2.70% -2.80%
1633 2.80% -2.90%
Twist Tests Standard Twist Reverse Twist
Digit 1 3.9 dB 8.0 dB
Digit 5 4.0 dB 8.4 dB
Digit 9 4.0 dB 8.2 dB
Digit D 4.2 dB 7.9 dB
Dynamic Range Test Dynamic Range
Digit 1 29 dB
Guard Time Test Min Pause Time Min Guard Time
Digit 1 Configurable 29.4 ms
Acceptable Signal to Noise Ratio Test
(-24dBV) pass 8.0 dB
(-18dBV) pass
(-12dBV) pass
TALK-OFF Test zero hits (pass)

Texas Instruments TMS320C6000 Resource Utilization

Tone generator

MCPS single-sample: 1.1
Program Memory: 2.5K bytes
Constants/tables: 900 bytes
Per-Instance Memory 64 bytes

DTMF detector

MCPS single-sample: 2.1
Program Memory: 6.5K bytes
Constants/tables: 340 bytes
Per-Instance Memory 456 bytes