V.34/Group 3 High Performance Fax Modem Family
Supervisory Tone Detection
Three parallel tone detectors (A, B, and C) are
provided for supervisory tone detection. The signal
path to these detectors is separate from the main
received signal path.
Each tone detector consists of two cascaded second
order IIR biquad filters. The coefficients are host
programmable. Each fourth order filter is followed by a
level detector which has host programmable turn-on
and turn-off thresholds allowing hysteresis. A pre-filter
and squarer precede tone detector C. This circuit is
useful for detecting a tone with a frequency equal to
the difference between two tones that may be
simultaneously present on the line. The SQDIS bit may
disable the squarer causing tone detector C to be an
eight-order filter. The tone detectors are disabled in
The tone detection sample rate is 9600 Hz in V.8 and
V.34 modes and is 7200 Hz in non-V.34 modes. The
default call progress filter coefficients are based on a
7200 Hz sampling rate and apply to non-V.34 modes
only. The maximum detection bandwidth is equal to
one-half the sample rate.
Auto-Dialing and Auto-Answering Control
Features are provided to allow the host to perform
auto-dialing and auto-answering. These functions
include DTMF or pulse dialing, ring detection, and a
comprehensive supervisory tone detection scheme.
The major control parameters are host programmable.
Transmitted Data Spectrum
The transmitter spectrum is shaped by raised cosine
filter functions, either the square root of 12.5% (V.34,
V.32 bis, V.32), 20% (V.17, V.33, and V.29), 50%
(V.27 ter, Bell 208), or 75% (V.22 bis, Bell 212A).
The transmitter output level is selectable from 0 dBm
to –15 dBm in 1 dB steps and is accurate to ±0.5 dB
when used with an external hybrid. The output level
can also be tuned by changing a gain constant in the
modem DSP RAM. The maximum V.34 transmit level
for acceptable receive performance should not exceed
Answer Tone (-D / -D90)
When the NV25 bit is a zero, the modem generates
a 2100 Hz answer tone at the beginning of the answer
handshake for 5.0 seconds (V.8) or 3.6 seconds
(V.32 bis, V.32, V.22 bis, V.22, V.23, and V.21). The
answer tone has 180° phase reversals every 0.45
seconds to disable network echo cancellers (V.8,
V.32 bis, and V.32).
The modem satisfies performance requirements for
received line signal levels from 0 dBm to –43 dBm
measured at the receiver analog (TIP and RING) input
in V.17, V.33, V.29, V.27 ter, and DTMF modes. In
V.90, V.34, V.32 bis, V.32, V.22 bis, V.22, and
V.23 modes, the receive level range is from –9 dBm
(–15 dBm at RIN) to –43 dB. (RIN is 6 dB lower than
TIP and RING.)
The carrier recovery circuit can track a ±7 Hz
frequency offset in the received carrier.
Echo Canceller (-D / -D90)
A data echo canceller with near-end and far-end echo
cancellation is included for 2-wire duplex V.90/V.34/
V.32 bis/V.32 operation. The combined echo span of
near and far cancellers can be up to 40 ms. The
proportion allotted to each end is automatically
determined by the modem. The delay between near-
end and far-end echoes can be up to 1.2 seconds.