More efficient modulation #115
Description
When using stereo the modulation of the signal is not very efficient where it looks more like a mountain than a plateau (most commercial stations have a plateau look on the RTL-SDR). This is not a huge issue but it most likely means more wasted energy in the transmission and therefore the possibility of less receive distance and/or quality. When the signal is modulated without stereo and rds and the deviation is set to 320000 and volume set to a good level (I know this seems high but it seems to make the actual modulation stretch around 130 - 140khz, which is the commercial standard in the US) it makes a really good looking plateau that uses the standard amount of bandwidth for commercial transmitters. In some cases, it even looks better than commercial transmitters! If this could be done with stereo and rds that would be amazing. The reason I do not use rds is because it makes the signal go past 200khz of bandwidth with this deviation level. If I do not use such a high deviation, the rds portion of the signal will stay inside 200khz but the main modulation plateau will be around 80khz wide, which is not ideal. Also, when using a deviation this high, you will need to reduce the volume, but it enables you to increase the volume and, in turn, increase the modulation bandwidth to a commercial level without adding distortion. To decrease spurious signals past 200khz, I will attach an image below of a relatively safe equalizer to use that prevents a lot of over modulation. You can increase or decrease the level of high frequencies depending on the songs you play, but this seems to work pretty well throughout genres. The large bass boost is simply because of PiFmAdv's default audio quality being mediocre, needing a lot more bass to sound similar to commercial stations. Trust me, the equalizer image below does not sound as bad as it looks, as for me the default audio in PiFmAdv has a lot of high frequencies and less low ones. You may need to make completely different equalizers than me, but this is what works in my case. I hope this helps the minority who actually care about the look and efficiency of the transmitted signal (me included). :)
