Stereo Generator User Manual
Digital Signal Pr ocessor
Stereo Generator
USER MANUAL
Page 1 May 2008
Stereo Generator User Manual
Quick Set up Instructions
(If not already fitted) Install the stereo generator at the rear of the transmitter
chassis in place of the “Mono” interface board. Connect 12 VDC from the
auxiliary power supply.
• Arrange to monitor the transmitter audio with a modulation meter or
spectrum analyser,
• Set the jumpers* for analog or digital input.
• Set the jumper for mono or stereo
• Set the jumper for pre-emphasis ON
• Set the jumper for 50uS pre-emphasis
• Select the required AGC mode (AGC off recommended initially)
• Turn on the transmitter and wait 20 seconds for the VCO to lock.
• Adjust the deviation control on the exciter board for 7.5KHz pilot tone
deviation. Alternatively set the level of the first pilot tone sidebands to 13dB below the carrier.
• Apply the incoming program in either digital or analog audio as required
• Adjust the level of the incoming audio so that the “limiting” led lights on
occasional audio peaks. The average level of analog audio should be
around 0dBm.
• Insert jumpers for desired AGC action. Each mode increases the audio
density.
*See overleaf for position of jumpers
Page 2 May 2008
Stereo Generator User Manual
Recommended Jumper Settings for Initial Use
The following jumper settings are recommended for normal operation:
i.e. Analog input
Stereo
50uS pre-emphasis
Peak deviation limiting (active at all times)
No AGC
NOTE: Restart generator by cycling power after changing a jumper setting. Ensures DSP
recognises changes.
Jumper Position Function Comment
JP1,JP2 IN Select AES/EBU input Jumpers can be left in
or out when analog
selected.
JP3 IN Select 3ID Input Jumper can be left in
or out when analog
selected.
JP4, JP5 OUT AGC off
See “AGC” section
JP6 IN Pre-emphasis time
constant
JP7 OUT Pre-emphasis ON
JP8 IN Mono/Stereo
Select
JP9 IN - Analog input
Analog or Digital
mode
Leave AGC off.
Over deviation limiter
active.
Selects 50 uS for
Australian standard
OUT -mono
Out Digital input
selected.
Page 3 May 2008
Stereo Generator User Manual
Specifications:
Model: Horizon DSP Encoder (R2)
Baseband Audio 20Hz ~15 KHz
Stereo separation > 50dB
Pre-emphasis 50 or 75 uS
Noise & Distortion Better than 0.01%
Analog Input level 0dBm
Analog Input Female XLR, (10K)
Digital Input AES/EBU XLR & BNC (3ID)
Output MPX BNC
Output Load 10K recommended
Maximum temp + 45C
Operating voltage 12VDC @ 200 mA
Input power 55 watts (nominal)
Size 2RU - depth 400mm
Page 4 May 2008
Stereo Generator User Manual
Overview
All Horizon rack mounting transmitters can be fitted with an internal stereo generator.
Generation of the composite stereo baseband signal is accomplished by a digital signal processor
(DSP). The DSP ensures accurate generation of the main, subcarrier and pilot tone. All critical
bandpass filtering is performed by the DSP as well as pre-emphasis.
The DSP also accepts AES/EBU and 3ID digital inputs. It will also accept domestic SPDIF
A digital “limiter” operates on the pre-emphasised output of the generator at all times. ensuring the
output signal is always constrained to 75KHz deviation. The limiter works by introdu cing a very short
delay in the audio path. Any signal peaks exceeding the maximum deviation are detected and used to
reduce the level of the delayed audio at the correct time.
The limiter is not intended to replace an external audio processor – it simply ensures that the transmitter
will not over-deviate when driven by excessive audio levels.
The generator also includes a simple AGC system with three levels of processing. AGC is primarily
intended for satellite rebroadcast applications however it is also useful where the transmitter is used
directly with a studio mixer.
Performance of the internal DSP processor cannot compare with dedicated external audio processors .
Adjustments
Apart from selecting the operating mode there are no adjustable controls on the generator. All functions
are performed in the digital domain under software control.
Multiplex output from the generator is at a fixed level with relation to the pilot tone. In the absence of
AGC the DSP will always constrain audio peaks to a maximum deviation of 75KHz .
Adjustment is confined to setting the pilot tone level for approximately 7.5 KHz deviation using the
transmitter deviation control and varying the level of the incoming audio at the external source.
The incoming audio is adjusted by observing the “LIMITING” LED on the stereo generato r board. The
LED should illuminate on occasional audio signal peaks for best dynamic range. The audio input can be
increased beyond this point by over 10dB with increasing loss in dynamic range. “Pumping” of the
audio will become noticeable at very high input levels.
Audio Levels
The analogue balanced inputs are set for nominally 0dBm, or 2Vpk-pk. There is 18dB headroom
allowed before the input circuit clips. The deviation limiter engages around +0dB, depend ing on stereo
content.
The digital path has its nominal level set for -18dB of full scale. This allows similar headroom to the
analogue path. The limiter then engages at approximately -8dB of full scale.
Output level for 75kHz deviation is 3.0V pk-pk.
Page 5 May 2008
Stereo Generator User Manual
LCD Deviation Display
The top line of the Horizon Transmitter LCD display is a digital “VU” meter. It shows the
average programme level radiated by the transmitter. The meter is only a guide to setting the
average deviation and should only be used when no other equipment is available.
============================= |
==========================================| ==
Correct audio level
on programme
Audio level too high
Adjust the audio level from the programme source to display an average level of
approximately 60~70% of the display length.
The small vertical mark is 75KHz deviation which enables the level to be set with fixed
tones. Apply equal level tones to the left and right channels at 0dBm in (0.774 VRMS).
Adjust the 10 turn deviation potentiometer on the exciter board to bring the LCD bar up to
the vertical mark. Pilot tone deviation should now be approximately 7.5 KHz
AGC modes
AGC can provide up to 12dB gain. Jumper settings set the headroom before the limiter
engages and compresses the signal. The reference is based on a decaying peak average.
Loud mode has the lowest headroom, resulting in maximum signal density and loudness due
to the hard limiter compression, but noticeable gain pumping on some program material.
Quiet mode typically allows for a larger dynamic range before the limiter engages.
AGC Jumper JP4 AGC Jumper (JP5) Mode
Out Out No AGC. Reference 0dBm
In Out AGC Normal Mode
Out In AGC Quiet Mode
In In AGC Loud Mode
“
NO AGC” is recommended for best dynamic range. Only the peak deviation limiter is
active. An external audio processor should be used for best quality audio. Ensure that the
“Limiting” LED only illuminates on occasional audio peaks.
“Quiet “ or “Normal” modes are recommended for satellite re-broadcast. It allows the
generator to accommodate level minor level changes in the downlink level without
noticeable impact on deviation or dynamic range.
“Loud” mode considerably increases the audio density at the expense of dynamic range. It is
not recommended for most broadcast applications.
Page 6 May 2008
Stereo Generator User Manual
Notes on DSP Operation
Audio Processing
The audio data initially passes through a 1st order IIR pre-emphasis filter. The filter can
have one of 3 time constants set by two jumpers- Bypass, 50uS, or 75uS. The characteristic
of the filter is within -0.5dB of the specified curve.
AGC is then applied if enabled. There are 3 levels of AGC headroom available. These are
determined by jumper settings.
The AGC system uses a peak hold detector, which has a decay rate of 1.5dB/s. The peak
signal is then fed into a 2nd order Butterworth low-pass filter . The cutoff period is set to ~3
seconds. The AGC is then determined from this reference output, with peak headroom
determined by jumpers.
When AGC is disabled, a fixed 12dB gain is applied to the AES/EBU reference.
Peak limiting is engaged when |L+R| + |L-R| after AGC exceeds the output clipping point.
This defines the maximum modulation level of a stereo signal.
When an overload peak is detected, the gain at the output of the buffer is ramped down
linearly until the clipped sample reaches the output at the maximum level. The output gain is
then restored at a fraction of the attack rate.
AES/EBU
The AES/EBU outputs are 5.0V level.
When the AESEBU is in bypass mode, the ADC signals are routed through the AESEBU
device.
If no AES/EBU is required the firmware detects the presence of the AESEBU device, and if
not populated, leaves the data stream non-inverted (assumes the links are in place).
Page 7 May 2008
Stereo Generator User Manual
WARRANTY
RF POWER (Manufacturer) warrants that this product is free of defects in both materials
and workmanship. Should any part of this equipment be defective, the Manufacturer agrees,
at its option, to:
A. Repair or replace any defective part free of charge (except transportation charges) for a
period of one year from the date of the original purchase, provided the owner returns the
equipment to the Manufacturer. No charge will be made for parts of labour during this period;
B. Furnish replacement for any defective parts in the equipment for a period of one year
from the date of original purchase. Replacement parts shall be furnished without charge, except labour and transportation. This Warranty excludes assembled products not manufactured by the Manufacturer whether or not they are incorporated in a Manufacturer product or
sold under a Manufacturer part or model number.
THIS WARRANTY IS VOID IF:
A. The equipment has been damaged by negligence, accident, act of God, or mishandling,
or has not been operated in accordance with the procedures described in the operating
and technical instructions; or,
B. The equipment has been altered or repaired by other than the Manufacturer or an
authorised service representative of the Manufacturer; or,
C. Adaptations or accessories other than those manufactured or provided by the Manufac-
turer have been made or attached to the equipment which, in the determination of the
Manufacturer, shall have affected the performance, safety or reliability of the equipment; or,
D. The equipment’s original serial number has been modified or removed.
NO OTHER WARRANTY, EXPRESS OR IMPLIED, INCLUDING WARRANTY
OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR USE, APPLIES
TO THE EQUIPMENT, nor is any person or company authorised to assume any warranty for the Manufacturer or any other liability in connection with the sale of the
Manufacturer's products.
Manufacturer does not assume any responsibility for consequential damages, expenses, or
loss of revenue or property, inconvenience, or interruption in operation experienced by the
customer due to a malfunction in the purchased equipment. No warranty service performed
on any product shall extend the applicable warranty period. In case of unsatisfactory operation, the purchaser shall promptly notify the Manufacturer at the address set forth below in
writing, giving full particulars as to the defects or unsatisfactory operation. Upon receipt of
such notice, the Manufacturer will give instructions respecting the shipment of the equipment, or such other matters as it elects to honour this warranty as above provided. This warranty does not cover damage to the equipment during shipping and the Manufacturer assumes no responsibility for such damage. All shipping costs shall be paid by the customer.
This warranty extends only to the original purchaser and is not assignable or transferable.
Page 8 May 2008
Loading...