WBDH AM STEREO TECHNICAL INFORMATION

"The Big 58" has been on the air on its present frequency since June 1, 2004. Our large omnidirectional antenna array is well suited for wideband AM Stereo applications and performs nicely.

In order to maintain monaural compatibility with existing radios we must broadcast our Left and Right signals (mono combined) on our AM transmitter in the conventional sense. However, this monaural mix, the L+R, must first be phase shifted by -45 degrees before it gets to the transmitter. It should be pointed out that this shift is completely transparent to the monaural listener; the shift is required for the receiver to properly re construct the AM Stereo signal.

Our ISB AM Stereo Exciter is the equipment we use to not only properly phase shift the audio components, but to effectively modulate all the AM stereo information onto our carrier signal (580 khz). In the exciter the first step is to generate the signal at 580 khz. We use a phase locked loop (PLL) circuit that can produce many frequencies from one reference crystal. The programmable divider is set to divide-by the proper interger to allow the voltage controlled oscillator (VCO) to lock to 580. This VCO has the proper loop filters to drastically reduce phase noise which would normally be a problem for AM stereo transmission. Next, the newly generated signal at 580 drives a phase modulator circuit. Quite frankly the phase modulator is the heart of the AM stereo exciter. It is used to phase shift the carrier about 1.5 radians  in accordance to the audio (modulation) applied to it. Just as the AM transmitter requires a mono mix (L+R) the phase modulator requires the stereo difference or L-R as its audio feed. Likewise, the L-R component audio must be shifted +45 degrees from 'where it was' in time. This is vital to ensure a quadriture relationship (90 degrees) between L+R and L-R. All of the audio shifting and matrixing that is required to generate the L+R and L-R component signals are performed in the first audio stages in the exciter. The individual left and right signals from the studio enter the exciter and are then matrixed into their component signals. The -45 degree L+R then goes through an amplifier stage and then exits the exciter for connection to the AM transmitter's audio input. In reality the L+R goes through an outboard equalization and processing stage before getting to the transmitter. This is vital to optimize the AM stereo separation. Before the L-R signal is applied to the phase modulator it too exits the exciter and then re enters it. The stereo difference channel must also go through an equalization and processing stage. The output of the phase modulator goes through a series of RF amplification stages before exiting the exciter. The complete phase modulated signal at 580 is in the order of about 500 milliwatts and this signal is now used to 'excite' or drive the AM transmitter.

WBDH uses a CRL matrix AM Stereo audio processor in conjunction with a 15 band equalizer for each audio component. Using our ISB modulation monitor and an AM stereo receiver, we can tailor the response curves to correct for group delays within the transmitter and thus optimize stereo separation. In addition our exciter sends out a 15 HZ 'pilot' tone, modulated piggyback onto the phase modulator. This pilot goes along for the ride in the event there are any ISB type stereo radios that require this tone to unlock the decoder. The Sony SRF series are 'forced stereo' meaning they do not need to detect the pilot to decode stereo. Our exciter derives the pilot frequency from the carrier reference crystal....10.24 Mhz divided by its internal fixed divider (16,384) = 625 HZ. This 625 HZ square wave is then divided by 42 and filtered to achieve 14.88 HZ (close enough). Interestingly enough our exciter is also capable of dividing the 625 HZ signal by 25 (dip switch select) giving 25 HZ which is the Motorola Cquam pilot tone. We don't use this mode or system- the only thing that would happen is the stereo light would light up on Motorola type AM stereo radios, and since our ISB system is 100 percent non-compatible with Motorola Cquam- no true stereo would be heard.

The output of our exciter is essentially a phase modulated carrier with no AM component. In other words if you were to listen to the signal produced by the exciter with the transmitter off you would hear a quiet spot at 580, and as you tuned off to the sides you would hear a faint audio signal. This is the L-R difference signal being heard. Since the output of the phase modulator has zero AM component you would hear nothing at center frequency. Now, if you listened to the exciter with a Sony SRF A1 Walkman you would hear the L-R loud and clear in the stereo mode and nothing in the mono mode. At this point you would still hear no stereo because the other half of the system is off- the L+R transmitter. Turning the transmitter on and un plugging the L+R audio will yeild a very strong carrier at 580 complete with the L-R audio and a faint 'flutter' from the pilot tone. Now, applying L+R audio to the transmitter you would get a full loud on air sound as expected, but now the quadriture relationship between L+R and L-R make themselves apparent; by patching out the left or the right channels going into the exciter this will produce a distinct drop in the 'dead' channel's sideband energy. By 'tweaking' the component audio gains and EQ bands it is possible to get about 30 db of stereo separation. Remember there is a 90 degree relationship between L+R and L-R, and if you draw it out vectorally you can see how one sideband goes away when that audio channel is removed. The SRF -A1 and A100 series are multisystem receivers, meaning you can select via a switch what system you want to decode....Motorola Cquam or ISB Kahn. When the WBDH system is received in the Motorola mode on the SRF, patching out audio channels into the exciter shows no stereo separation at all. This is normal since our system is a FREQUENCY SEPARATED system (IE sidebands) whereas the Motorola system is a PHASE SEPARATED system and thus is subject to many problems that our system is immune to.