Distortion in music through fm transmitter

[IndianapolisFire917]

I'm a second year lor user. This year I have a 100mw/500mw fm transmitter. I'm getting distortion (best way I can describe it) only on certain songs. This transmitter is Chinese and didn't come with any directions. It has a digital LCD screen to select the channel. A left knob for music volume and right knob for microphone volume. I have no idea how to select the 100/500 output. So if anyone has any experience with this please chime in. But my bigger problem is how certain songs come out of the transmitter. Some are perfect, others sound muffled or not all of the music is coming through. The music is all mp3 format but at different rates ranging from 192 up past the 300s. It comes right out of the laptop using a 3.5mm jack ran roughly 15 feet to the transmitter. The audio line is clear of obstruction and electrical lines. I'm dumbfounded. The computer volume is at 25% and transmitter volume is 75%. I'm just guessing it's set at 100mw. There are no issues if I run the audio cable directly to an amplifier so I can eliminate the laptop itself. Looking forward to responses. Thanks everyone.

[ItsMeBobO]

removed  repeat

Edited December 15, 2014 by ItsMeBobO

[portcity_gt]

Try 35% on the laptop and 45% (11 o'clock) on the transmitter.

[Max-Paul]

In other words turn down the laptop or music volume control some. Sounds like some of the songs are a bit louder. Does this hold true for those songs that sound bad? Might try downloading a free program called Audacity or MP3gain. Both of these programs allow you to adjust the max (peak) volumes of songs. And I am thinking that MP3gain might allow you to select a folder and either increase or decrease the volume of all songs. Very helpful when you do a show that all of the songs have the same volume. Dont have to keep adjusting the radio volume while in the car watching the show.

[dstevens]

 And I am thinking that MP3gain might allow you to select a folder and either increase or decrease the volume of all songs. Very helpful when you do a show that all of the songs have the same volume. 

 

Absolutely, MP3Gain is the best tool to adjust an entire directory.

[MSB]

It is definitely over modulation. That means the sound input into your transmitter or your transmitter volume is too high. I made a receiver and use SDR tough on my tablet that lets me see the spectrum and adjust my "volume" accordingly. It works great and you can do this for less than $30. Trust me, it's a whole lot easier than guessing!

[Oliver Blatt]

I run my computer at 100% and my transmitter at 30%. Crystal clear audio every time. But, I did process all my audio files to be "normalized" before programming sequences. Crystal clear audio with no issues. I found the quality of the sound degraded sending a lower volume to the transmitter. But, like others said, turn down the transmitter and turn up the volume of the computer.

[k6ccc]

One advantage to running a high level from the computer and lowering the input level at the transmitter is that a higher level in the cabling will generally result in less noise getting into the cables.  In my case I'm running almost 200 feet of cable between the sound mixer next to the show computer and the transmitter.  Running it over balanced cables with isolation transformers at each end.  Working fine...

[tmcmillin2001]

If it's sainsonic brand, the way to toggle to (H) high output is hold power button as you plug in power supply.... display will be L, use arrow so switch to H, then hit power button again...

[chandalen]

As mentioned above, put laptop to 100% as long as its not distorted on the laptop speakers. 

Put the transmitter at the lowest possible amplification volume to sound like a normal station in a car at the cars volume level. 

(people should not have to crank up the volume knob to 40 to hear the music, they also should not have to turn it down to 3 because its so loud)

 

Due to FCC rules, some transmitters come with vastly underpowered power supplies.  Look at the input voltage and power, probably 12vDC and 500-800mA.  You could use a different power adapter at same voltage, higher amprage (like 1-1.5A)  The device will only use as much power as it can, its too much voltage that will blow it up.

 

If you have humming, or odd static, ball the wires up in your hand, or touck the metal base of the antenna, if the sound goes away, head to radio shack and buy a ferrite trap and install it on the power cord / line in from the laptop.

[Santas Helper]

As mentioned above, put laptop to 100% as long as its not distorted on the laptop speakers. 

 

You need to know what the gain is for each song before cranking the laptop at 100%.

 

The gain varies from song to song (mp3 to mp3).

Alot of songs, the gain level will possibly need to be adjusted down to around 80% if not lower to keep from distorting on the computer.

So first, figure out the gain level of each song (I use MP3Gain) then adjust the computer untill you have distortion free music. then adjust the transmitter if it has that capability.

[Max-Paul]

Santa,

 

Aint that sort the purpose of MP3Gain? To analyze and then adjust all of the songs in a folder so that they all are at the same level? Or this is not what MP3Gain does?

[Santas Helper]

Yes MP.

 

That's one of the purposes for MP3Gain. You can do one song or a whole folder to the same settings at once.

 

But if you don't know about it or don't lower the gain in the songs that may be high, then that song may be distorted without even knowing about it.

I use MP3Gain when ever I download any and every song. This way I know for sure where my audio gain stands.

Audacity helps to visually see how the peaks are in songs so I use that as well for a visual.

[DocBrown86]

I'm getting static in one song, and only on words with "s" and "t". Ie) Chris"t"ma"s", the t and s create a burst of static. I have checked the gain, and it's the same as the other 3 songs. I even lowered it and removed some bass using Audacity just to be sure. Static remains. What else could the issue be? Never had this happen in 7 years.

[k6ccc]

Short version most likely is that the high frequency components are exceeding the deviation limits of your receiver (your transmitter is just passing along the audio that is presented to it).  If you want I can get into a discussion of pre-emphasis, but it's not needed...

Bass is not likely the problem, the high frequency harmonics in the "S" and "T".  since you know how to use Audacity, there is an effect for fixing that.  I believe it's called "De-essing" or something like that.  If not that, then use a low pass filter to reduce the high frequency levels a bit.  Be gentle with a low pass filter however.  Try a little (fairly high cutoff frequency and only a couple dB of reduction) and if that does not eliminate the problem, reduce the cutoff frequency a little and do it again.  Each time start with the same original file rather than process the same file over and over.

[DocBrown86]

Thanks Jim, that solved the problem!