I used a Tektronix 465B oscilloscope to calibrate the tilt of a 60 Hz square wave but I still have overblown base. I must drastically reduce the bass to prevent overmodulation, although the sound somewhat improved from the pre-calibration state.
When I used the oscilloscope, I made sure it was set to DC coupling and used a Y adapter so I could have an input going into the scope and the transmitter at the same time. Removing the transmitter drastically changed the shape of the 60 Hz wave. I went with the calibration with load on the transmitter input as I was under the impression this was the correct method.
Just wondering if anyone can shed some light on my excessive bass issues and if I might have done something wrong.
I also used a quick sweep tone to make sure I didn't have any HF rolloff. The shape was perfect.
Taking a wild guess here but it sounds like there is an impedance mismatch from the audio going into the transmitter and what it wants to see at its input.
Most industry standard stuff is 600ohms and around 1vrms "line level", but it can vary from some manufactures to others, or consumer audio to professional audio equipment. Properly designed equipment should have a reasonably high impedance at its line level input, often at multiple k-ohms so not to drag down the audio voltage of what is plugged in and cause excessive loading on its audio buffer circuits.
If you have a spare tape deck or EQ (with bypass button on) or something that will pass audio through buffered without changing audio tone I would try that between the transmitter and your source audio. If that clears it up then it is obvious that audio buffering will be needed, or your audio source is not able to match the impedance of your transmitters audio input.
Not sure if you are going directly from a computer sound card to transmitter but that can cause issues also because most sound cards to not play nicely with heavy line output loading, or at least in my experience with cheaper PC cards.
Thank you for the response and sorry to keep picking your brain, but would an impedance mismatch cause a severely distorted 60 Hz square wave when I connect the output of my radio (on-air transmitter audio) to the oscilloscope? What I'm seeing is this. I start a 60 Hz square wave in my audio processing. Then I tune my FM radio to my radio station and plug its output into the oscilloscope. The oscilloscope then displays a severely distorted wave with the left edge being way too high and the right edge being way too low. Tilt control isn't enough to correct this. I'm not sure if the audio coming out of the radio should show a perfect wave or not, but it's perfect when going directly from the sound card to the o-scope
The reason I ask this is because if I get an EQ or something to pass buffered audio through, I want to be able to measure if the problem got corrected by once again, connecting my radio audio to the oscilloscope and verifying that the wave now looks correct.
You can not use a radio as an accurate on-air modulation monitor. Sure it is fine to use for in-house listening by the DJs but that is about it. Besides that, passing a square wave through just about any kind of audio amplifier, let alone a radio transmission, is going to deform the waveform some because of the natural circuits roll off of either high or low frequencies. Sure you can connect an oscilloscope to the audio output of a radio but it will not show you anything useful for testing purposes of monitoring a transmission because most radios themselves have circuitry that will modify the audio before it hits its speaker output.
The only thing your scope is going to be useful for at this point is testing frequency response of the audio going into the transmitter with either the jacks connected to it or disconnected and testing the audio source itself to compare against, and hopefully discover which piece of equipment is not handling the audio properly.
Are you using any pre-emphasis on the transmission? Does the transmitter have it built in for you without external processing? Without it you will not get proper emphasis on the high audio frequencies which would make the sound muffled and have too strong of a low bass end.
There are so many factors that could come into play with your setup. I would just take the transmitter itself on a dummy load and test multiple audio sources into it to rule out if it is the transmitter itself, or if it requires something special to sound right. If that tests okay, then move on to the audio source and work backwards one piece of audio equipment at a time in the chain to see where the problem begins.
The transmitter has built in pre-emphasis (75 ms) with no option to disable it. I wish this was not the case. To compensate for this, I select 75 micro seconds and the corresponding de-emphasis in my audio processing to get straight audio. I have heard pre-emphasizing and de-emphasizing again can cause a loss in loudness and quality, but not sure if this applies in my setup.
I know the problem is with the transmitter and not the audio source. I recently set up a small station for a friend using similar equipment and the same audio processing, but a different transmitter. The result was loud, rich, punchy sound with perfect stereo separation and no audible over-modulation. It was actually slightly louder than commercial radio stations and sounded amazing. I had this same transmitter on the same audio source I'm using now with similar results, but the output final died. Therefore, I wonder what the problem is with this new transmitter. Perhaps it's made with cheap components.
I am using a ground loop isolator audio cable from radio shack to feed the transmitter. I use this to filter out noise generated by the internal components of the computer and surrounding equipment. This distorts the square wave, but if I swap it out for a regular audio cable, the sound is the same on the radio.
Also I tried running the audio through an equalizer with the EQ bypass button turned on. This didn't filter out the overblown base. Looks like I might just have to live with this problem. The audio I have now really isn't that bad, but is just slightly less than commercial radio which bothers me.
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