I lowered my reference level and the audio seemed to come alive with great stereo separation and clarity. Having anything other than 75 micro seconds and de-emphasis checked in the audio chain results in severe over modulation. I've tried leaving de-emphasis off and setting the audio chain to 15 or 25 µs with the thinking that the transmitter may be set to 50, but it causes severe distortion. Therefore, unlike other Chinese transmitters, I believe this is actually set to 75 µs, but still would like to disable it anyway.
If I open up this transmitter, or connect to the RS-232 serial port on the back, would there be a way to disable the built in pre-emphasis? I'm struggling with muddy, lackluster sound and believe this is the cause since I am also de-emphasizing in my audio chain.
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.
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.
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.
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.
Thanks for the reply Kage. However, I found the RDS port to be basically useless. It introduces all kinds of high frequency static and noises/beeps from the computer into the audio chain. I ended up using a separate, standalone instance of Stereo Tool to send the info to the main sound card. I then have breakaway running with instance of Stereo Tool as a dst plugin. This takes care of the stereo widening and audio processing. Hence, I now have processed audio and RDS going into the main inputs of the transmitter from the same sound card. This seems to work very well.
I now have a pretty sweet setup using a very nice ground plain antenna, a 12 db attenuator and the transmitter loafing along at the 0.0 watt setting. SWR is damn near flat 0. The signal goes about halfway down my street. Just enough for a decent signal around my house but not enough to attract major attention (hopefully).
Would you recommend using a separate sound card for RDS and can you recommend a simple program other than Stereo Tool to send RDS text to an output? I'm currently using stereo tool as a dst plugin and multiple instances running at the same time tend to cause problems.
I was able to get the RDS to work through the regular line in with normal sound, but this transmitter sounds absolutely terrible. The sound is so lack-luster and muddy. I had a cheap $100 transmitter that competed with commercial radio stations but the output final finally died. Hence, I bought this expensive unit but it sounds so bad. Comparing it to cmmmercial FM audio is a comedy act. Is there something wrong in my audio chain or is this just a POS?
I purchased a CZH-T501 transmitter and have attempted to use the RDS port on the back. I am able to get RDS to go through to the radio but the sound is distorted and high pitched with no bass or mid range. I have tried to use the pre-emphasis settings in breakaway broadcast processor and stereo tool, but it never sounds right. Am I correct in my assumption that this port is for RDS only and not designed as a multiplex port for proper audio? I am able to encode the stereo but the sound is too high pitched. Is there a way this can be fixed?
I was using a dummy load. But now I'm using a radio shack adjustable power supply with a ground plane in the attic. When I turn the voltage down the range drops from several miles down to about 700 feet. Even though my transmitter calls for a 6 amps the 2.5 amps seems to work just fine.
Thanks for that info. I will definitely make my own 3.5 mm to BNC connector. I hope you don't mind my asking one last question, but would you think that transmitter I linked above (which states 0 to 50 watts) would heat up if I lowered the power output? I had one cheap transmitter that would get really hot and I wonder if the more expensive ones have a better way of reducing the power without generating heat.