tony sayer wrote:
In article , Frank Johnson
writes

Dave Plowman wrote:

In article ,
Rembrandt Q. Einstein wrote:


So, would I be correct in assuming that the FM output of the transmitter
is generated directly from the digital distribution signal, without
passing through a DAC first?


No - the FM transmitter is an analogue device so the NICAM distribution
system will have a DAC at it.


For all the national services, the programme chain at the transmitter
site is:
Energis fibre or Microwave link feed, 8.448Mb/sec
Demultiplers down to 4x 2048 kbit/sec
"RE" electronics NICAM decoder, each national service has a bitrate of
676Kbit/sec for the stereo pair.
Nicam decoders provide L+R analogue output.
Several metres of balanced lines from the NICAM bay to the "Transmitter
Input Equipment bay"
SBS Electronics "MPX5" stereo coder. RDS data is added at this point.
TM4 series RF drive where the Frequency Modulation is applied.
High power amplifier.

Frank.



Who makes those TM4 series drives Frank?..
TM4/2 and TM4/6 are BBC design (Now obsolete), probably manufactured
under licence by Eddystone Radio. They are crystal controlled,
frequency modulation is applied to a low frequency oscillator which is
mixed with the crystal oscillator. A frequency control loop stabilises
the final output frequency. This has a time constant of around 30
seconds so as not to affect the modulation.

Frank