RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
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Function 25:
Answer Tone Length
Acc. Method: 2
Addr.: 229h, 228h
Function 26:
Silence After Answer Tone Period
Acc. Method: 2
Addr.: 22Ah, 22Bh
The ITU-T 2100 Hz answer tone length and silence after answer tone are calculated as follows:
Configuration
Equation
V.8, V.32, V.21, Bell 103
N = T x 300 (V.32 silence: N = T x 2400)
V.22 bis, V.22, Bell 212
N = T x 600
V.23/75Tx, V.23/1200Rx
N = T x 75
V.23/1200Tx, V.23/75Rx
N = T x 1200
Where:
N is the decimal value of the hex number written to RAM and T is the time is seconds.
The MDP will rewrite the default values when DTR is turned off or the NEWC bit is set.
The end of answer tone transmission may be determined by monitoring bit 051h:3. This bit will be set to a 1 when the
answer has finished and the silence period has commenced. Unless a power-on reset is performed, this bit must be reset by
the host if it is to be monitored again on the following connection.
Note: Address 229h, 228h lengthens individual phase reversal times in V.8/V.32 bis/V.32. The answer tone length may be
adjusted by increasing or decreasing the number of phase reversals at address 04Bh. The default value at address 04Bh is
08h (8 phase reversals). This value may be changed only after DTR is set.
Functions 27 - 29: Tone Detector Bandpass Filter Coefficients
Acc. Method: 3
Addr.: See Table 4-4
A block diagram of the three tone detectors is shown in Figure 4-6. Tone detector C is preceded by a prefilter and a squarer.
The purpose of the prefilter and squarer is to allow dual tones to be detected while rejecting the main channel energy. For
example, TONEC can be programmed to detect a difference frequency generated by the squarer for detection of 350 Hz
and 440 Hz. The prefilter would be designed to reject the energy in the 600 to 3000 Hz band. If the dual tone pair of 350 and
440 Hz appeared (or any other frequency pair in the range of 300 to 600 Hz with a difference of 90 Hz) TONEC would turn
on.
The SQDIS bit (02h:6) allows the squarer in front of tone detector C to be disabled. If the squarer is disabled then tone
detector C will have four cascaded biquads (since there is a prefilter consisting of two biquads), forming an 8-order IIR filter
with user programmable coefficients. To make the prefilter transparent (to use TONEC as a 4th order filter), write 7FFFh in
coefficients A1 and write 0000 to all other biquad coefficients.
The implementation of the filters allows user definition of the characteristics of the prefilter and the three tone detectors.
Table 4-4 provides the DSP RAM address codes for the filter coefficients. Table 4-5 shows the default values. Figure 4-6
shows that the prefilter and the main filter sections of the tone detectors are fourth order (two second-order biquads in
cascade), thereby allowing a wide variety of filter characteristics to be synthesized. The only limitation on these user-
definable shapes is that their gain should be around unity at the pass frequencies to avoid problems of saturation at one
extreme (gain too high) and digital noise at the other (gain too low). Computation of the filter coefficients can be performed
by any infinite impulse response (IIR) filter design program which outputs the coefficients in cascaded second-order
sections.
The default sample rate is 7200 Hz, however, in the V.8/V.34 mode, the sample rate is changed to 9600 Hz and all filter
coefficients are changed by the MDP. A soft or hard reset is recommended after a V.34 connection to restore default filter
coefficients.
The level detector in each of the tone detectors flags the detection of a tone if it is in the tone detector passband and if it is
above an upper threshold defined by THRESHU. The tone detected flag will remain set until, or unless, the tone falls below a
lower threshold defined by THRESHL.
The tone detectors are preceded by an AGC. The gain of the AGC may be read at address 8B9h (DUGAIN). By default,
DUGAIN reaches its maximum gain, at a value of 7FFFh, when the receive level is -26 dBm or lower. Signal levels below -26
dBm are thus not affected by the AGC, therefore, the threshold comparator will see a decreasing signal level as the input
signal is lowered. The THRESHU and THRESHL adjustments are limited to signal levels below the AGC cut-off point (-26
dBm). If the THRESHU value is adjusted to try to limit the detection threshold to -20 dBm, the AGC will not allow it.
To raise the AGC cut-off point, decrease the value in address BBB (DAGCRF). The DAGCRF default value is FF00h, which
results in the -26 dBm cut-off. Decreasing DAGCRF to FE00h raises the AGC cut-off to -20 dBm, thus allowing THRESHU to
be adjusted for a minimum detection level of -20 dBm or lower.
Summary of Contents for RC336DPFL
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