Motorola 6864115B62-C Detailed Service Manual page 93

Controller Board Audio and Signalling Circuits
2.3.1 Sub-audible Data (PL/DPL)
Sub-audible data implies signalling whose bandwidth is below 300Hz. PL and DPL waveforms are
used for conventional operation and connect tones for trunked voice channel operation. The trunking
connect tone is simply a PL tone at a higher deviation level than PL in a conventional system.
Although it is referred to as "sub-audible data," the actual frequency spectrum of these waveforms
may be as high as 250 Hz, which is audible to the human ear. However, the radio receiver filters out
any audio below 300Hz, so these tones are never heard in the actual system.
Only one type of sub-audible data can be generated by U0221 (ASFIC CMP) at any one time. The
process is as follows, using the SPI BUS, the µP programs the ASFIC CMP to set up the proper low-
speed data deviation and select the PL or DPL filters. The µP then generates a square wave which
strobes the ASFIC PL / DPL encode input LSIO U0221-18 at twelve times the desired data rate. For
example, for a PL frequency of 103Hz, the frequency of the square wave would be 1236Hz.
This drives a tone generator inside U0221 which generates a staircase approximation to a PL sine
wave or DPL data pattern. This internal waveform is then low-pass filtered and summed with voice or
data. The resulting summed waveform then appears on U0221-40 (MOD IN), where it is sent to the
RF board as previously described for transmit audio. A trunking connect tone would be generated in
the same manner as a PL tone.
2.3.2 High Speed Data
High speed data refers to the 3600 baud data waveforms, known as Inbound Signalling Words
(ISWs) used in a trunking system for high speed communication between the central controller and
the radio. To generate an ISW, the µP first programs the ASFIC CMP (U0221) to the proper filter and
gain settings. It then begins strobing U0221-19 (HSIO) with a pulse when the data is supposed to
change states. U0221's 5-3-2 State Encoder (which is in a 2-state mode) is then fed to the post-
limiter summer block and then the splatter filter. From that point it is routed through the modulation
attenuators and then out of the ASFIC CMP to the RF board. MPT 1327 and MDC are generated in
much the same way as Trunking ISW. However, in some cases these signals may also pass through
a data pre-emphasis block in the ASFIC CMP. Also these signalling schemes are based on sending
a combination of 1200 Hz and 1800 Hz tones only. Microphone audio is muted during High Speed
Data signalling.
2.3.3 Dual Tone Multiple Frequency (DTMF) Data
DTMF data is a dual tone waveform used during phone interconnect operation. It is the same type of
tones which are heard when using a "Touch Tone" telephone.
There are seven frequencies, with four in the low group (697, 770, 852, 941Hz) and three in the high
group (1209, 1336, 1477Hz).
The high-group tone is generated by the µP (U0101-44) strobing U0221-19 at six times the tone
frequency for tones less than 1440Hz or twice the frequency for tones greater than 1440Hz. The low
group tone is generated by the ASFIC CMP, controlled by the µP via SPI bus. Inside U0221 the low-
group and high-group tones are summed (with the amplitude of the high group tone being
approximately 2 dB greater than that of the low group tone) and then pre-emphasized before being
routed to the summer and splatter filter. The DTMF waveform then follows the same path as was
described for high-speed data.
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