Sound Quality of Music Through House FM Transmitters

Just to see how these things work, I purchased this inexpensive Sain Sonic CZH-05B FM Transmitter. It appears to work just fine.
I tried to do some direct testing of sound quality by transmitting an uncompressed wave music file of the same song playing on the professional radio station just to compare, and I also then used an audio splitter to compare transmitted sound to same sound fed into the receiver with cables.

I do notice the sound quality difference. The transmitted sound is clear but definitely inferior.
My question is about how these things work, can transmitted sound approach the quality of direct connection?
Can transmitted sound approach the quality of the sound coming from a professional radio station playing exact same song?

Would a more expensive FM transmitter have better sound quality and if so what should people look for when buying them?
Two things about my question: This is for a rural area with no neighbors in sight, plus the question is about signal quality, rather than range. Thank you for your advice.

For my home FM station I use the ZFMSM FM modulator.
https://www.tonercable.com/pdf/zfmsm_5872_agile_fm_stereo_modulator.pdf
This unit is designed for flat audio response so the audio signals must have pre-emphasis for proper FM receiver sound. Sadly this unit has been Discontinued but there are other professional quality modulators on the market. That said, don't expect high quality sound from even the best RF modulator due to the limits of stereo FM channel requirements. I use an old YAMAHA rev 7 to jazz up the source a bit before broadcast so I don't expect the OTA sound to be the same.

https://en.wikipedia.org/wiki/FM_broadcasting#Pre-emphasis_and_de-emphasis

https://flic.kr/p/2boTNVv https://flic.kr/p/2cH9Sa7

I understand that you are saying that even a more expensive FM transimitter will not be as clear as direct connection or a professional radio station signal.

The transmitted sound is noticeably worse on the transmitter I have so if the goal is to get better sound, should I instead consider something like a wireless HDMI module to send sound from computer to receiver without using a wired connection? Would quality of sound through HDMI wireless modules be better than using any FM transmitter?

It's really common for people to over-modulate the transmitter into signal distortion. To prevent this you need a limiter/compressor for the FM transmitter audio inputs to improve sound while increasing the deviation to improve receiver signal power. The uncompressed audio is not very transmitter friendly.
Using GQRX and RTL dongle

Notice how the commercial FM channel 103.3 looks like a frequency sweep box because of music compression while my home station at 103.7 is a lot more peaked around the carrier without much compression.
Don't know much about HDMI wireless modules because I hate HDMI as a standard starting with the stupid connectors.

The test Setup consists of plugging this splitter into a PC Laptop then the black cable goes directly into the Receiver and the white cable goes into the FM Transmitter, as you can see.

If you then switch between Receiver's inputs (tuner vs direct connection) every five seconds, you can then clearly hear the difference between transmitted sound vs. direct connected sound.
This definitively illustrates the question. How can I get the quality of both to be similar.
Can you listen to this attached Test1.mp3 file and tell me what you think?

I didn't listen to your file because it won't provide any useful technical information with my ears vs yours and different systems.

Well if I could ask you or anyone else reading this thread to listen to that file which provides 5 seconds of one source immediately followed by 5 seconds of the other, maybe I could at least get an opinion...

I listened and heard:
1) Stereo
2) Left channel only
3) Stereo
4) Left channel only.
My computer speakers are a little better than cheap ones but there is just a 3" one-way speaker in each one.

c627627 said:

I understand that you are saying that even a more expensive FM transimitter will not be as clear as direct connection or a professional radio station signal.

The transmitted sound is noticeably worse on the transmitter I have so if the goal is to get better sound, should I instead consider something like a wireless HDMI module to send sound from computer to receiver without using a wired connection? Would quality of sound through HDMI wireless modules be better than using any FM transmitter?

Click to expand...

a digital interface could be better than fm analogue transmitter, if your requirement is to link your audio inside the house wirelessly then digital is better. I have one transmitter and receiver that links through Bluetooth stereo, get professional sound quality.
I have worked with a similar transmitter as you have, I do not remember any issue of sound quality. you can do another check by providing audio through mic input with low input level. I doubt this transmitter to have an audio pre-emphasis .

As others mentioned you might be over modulating the transmitter, or the band width of the transmitter might be having any issue. Also check the FM receiver, there are bad receivers too. sorry I could not hear the audio for any comment about it.

mbarazeen said:

I doubt this transmitter to have an audio pre-emphasis.

Click to expand...

Yes it sounds like it has the proper pre-emphasis. If it didn't then it would sound muffled like an old AM radio.

there are a few things that will have an effect on the sound from the transmitter. as everybody has mentioned, the pre-emphasis is one thing, but according to AG, your transmitter seems to have that already. some other things are a) input level, if you are overloading the input you will have distortion... b) deviation, if the transmitter's deviation is wider than a commercial FM transmitter, you will get distortion (same type of clipping effect as overloading the modulator input)... c) off frequency, if the center frequency of the transmitter is offset from where it should be, you will get distortion (off freq transmitter will tend towards even harmonics in the distortion)... d) nonlinear modulator response, the modulator might have nonlinear portions in it's voltage to frequency curve (this happens when the varactor diode in the modulator is operated near zero bias, the capacitance change gets nonlinear when the diode is approaching conduction, and so the frequency changes at a different rate than when the diode is reverse biased), a nonlinear diode response will produce even harmonics in the received signal. also, be aware that in order to keep the 19khz pilot tone from interacting with the original audio, the audio input is bandwidth limited to 15khz in an FM transmitter.

Thank you for posting. There is a lot of useful information in how these things work on paper.
But the audio example I posted is the best way to explain the thread question. (If you can ignore the fact that one channel appears to be lower than the other) the actual clarity of sound difference can be heard in the Test1.mp3, *IF* you use headphones or really good speakers.

So the question is really: if the transmitter complies with everything you posted, will the transmitted sound be as "good" as the direct connected sound? This files alternates 5 seconds of one vs the other, and really what I am asking is will buying a more expensive transmitter achieve that goal of the first 5 seconds of this file being similar to the second 5 seconds, again, the difference can be heard clearly on headphones or really good speakers.

Maybe you are young enough or are old but have excellent hearing aids like me to still be able to hear sizzling high frequencies between 15kHz and 22kHz.
Those frequencies that are not transmitted or received on FM stereo are less than one octave but if you can hear them on the original recording then you notice them missing on the radio. My hearing is normal for my age (69 when I got my hearing aids 3 years ago) and here is a graph showing normal high frequency hearing loss with age:

I cannot hear the difference on low quality computer monitor speakers.
I can hear the difference on high quality home stereo speakers. The question is about listening to this on high quality home stereo speakers.
I would assume anyone with reasonable hearing can tell the difference on their home stereo, but maybe I'm wrong.

Many cheap home stereos sound awful. Many people are tone deaf (don't like music) and buy a cheap stereo for parties.