I have a pretty complicated setup. Things sound good and modulated correctly, but something is just a bit off that I can't quite put my finger on.
Here's what I have:
FM audio processor reference level: -3.5 db (60 Hz square wave tilt and coeff adjusted with an oscilloscope - dc coupling turned off) Stereo Mix and input sound card: 0 dB Output sound card: -23.2 dB Aphex 10/4 interface: +6 dB per channel FM transmitter audio setting: +6 dB per channel
I have 1/4 analog audio from the computer to RCA input on the Aphex. Then XLR from the Aphex to the FM transmitter.
The FM transmitter has dip switches for 600 ohms and 10k ohms. Currently set to 10k as 600 ohms results in slight loss in highs. FM transmitter pre-emphasis is turned off; pre-emphasis handled at the audio processor. Fm transmitter set to stereo - (using L, R, or Mix input resulted in loss of volume with mplex output).
The Aphex has a button for 600 ohm termination resistor which is turned off, but makes no difference in sound either way.
The output sound card doesn't seem to like being turned up too loud with the input in the FM audio processor being set as it is. If turned up too much, it will distort then disconnect from the OS. That's why I have the Aphex and Transmitter set to +6 dB to add the extra boost.
I'm just wondering if I'm doing something wrong. I get constant clip warnings in the FM processor, but it doesn't seem to be audible or cause any issues. The output sound card in Windows Control Panel pings at the last (uppermost) bar with fluctuation up and down (3 to 4 bars). This seems desirable to me.
I have different software running in the background to output RDS to the same sound card. It makes no difference in sound weather it runs or not.
End result is very slight loss of luster compared to commercial FM. Stereo separation is great and the RDS comes through instantly. Any tips on levels would be appreciated.
Have you tried other audio sources besides your computers sound card? Also what computer or sound card is this? Some cheap tablet computers and even laptops have headphone out only and will sometimes "disconnect" in software when it doesn't detect a headphone load. Even worse some have all in one jack connectors that are purely software controlled to switch from headphone/line level and can sound terrible.
The majority of time people experience these issues is when driving pro audio gear with consumer line level audio cards where things start to clip or simply doesn't have enough drive power requiring a preamplifier which I'm guessing the Aphex is used as? That a mixer?
Make sure you are using line level through and through and not headphone output. Headphone outputs are designed to push small loudspeakers at 8-600ohm and will not match the impedance or noise floor of professional line level inputs. The receiving source of line level should always be around 10x the impedance of the driving source or you could experience high or low frequency roll off from loading down the source. There used to be an exception to this with old equipment and heavy line level drivers using analog transformers but any modern equipment the rule should apply.
Watch out for differences between consumer and professional line level voltage differences. Usually if it's XLR or balanced lines it will be pro level, any TRS and RCA connectors are usually used with consumer levels in mind and there are converters and preamplifiers to go from one to the other, or if lucky your equipment will have a padding switch on inputs and/or outputs.
I'd connect up a dummy load, run lowest TX power so you can monitor without broadcasting and try a few other pieces of audio gear to hear what in the chain is having the most drastic effect. Other than that it might simply be your audio processors settings which when everything else is setup properly will have the strongest effect on audio presence and loudness. This of course is where you customize your sound for either light or aggressive processing, punch, compression and limiting which can have huge effects on the overall listening experience.
One last thing.. what are the clip lights for? Overload protection lights or are we talking about hard limiting (clipping) used to increase perceived loudness? Huge difference between the two. Would help to know if this is a separate processor or something built into the FM TX. The latter of which often is lackluster in most cheaper all in one units.
Thank you very much for the response. I tried a hooking a bluetooth receiver connected to my phone directly into the Aphex. With it being unprocessed audio it sounded horrible. Unfortunately, I only have FM processing on the PC I'm using. The Sound card is an onboard Realtek (192 KHz) with a regular 1/8 inch speaker out jack. All the other jacks are either for surround sound speakers or a microphone. The jack I'm using shows as "speakers", rather than "headphones", in Windows Sound. I plugged some headphones into it as an experiment and it still showed speakers.
I believe you're correct that the issue is trying to drive pro gear with consumer-based hardware. I'm using the Aphex as an amplifier but it seems to fall just short of doing the job adequately. Do you have any recommendations on computer sound cards that might be better suited to run a radio station?
That's interesting regarding the impedance. Perhaps that would explain why I experienced inadequate highs when setting the transmitter dip switches to 600 ohms rather than 10000 ohms.
I was actually overmodulated. I lowered the sound card level which fixed that issue. Then I changed some settings in the audio processor (Breakaway Broadcast Processor) for a more aggressive, punchy sound. It's still not 100% but much better. I'm not sure what the clip warning is for. I believe it has something to do with the sound bars in Windows Sound pinging to maximum. The transmitter doesn't have any built in processing other than pre-emphasis, which is disabled, and stereo (enabled).
Post by dave radio on Apr 25, 2020 10:38:40 GMT -6
Always interesting to see how others arrive at their system chain for FM broadcast. I'll share mine.
* I don't use computer internal sound card, instead coming out of computer into an older Tascam US-144 Mark II USB audio interface. (Cheap & works quite well)
* RCA out (-10db) on the Tascam goes into Henry Engineering "The Matchbox" to raise to +4 operating level so that...
* I hit a good input level into an Aphex Compellor which goes into an Aphex Dominator II.
* Dominator II output goes +4 back into The Matchbox to reduce back to -10 output so that I don't overload the RCA inputs on my (cheap) made in China xmtr.
While not as loud as commercial FM, I, in my little neighborhood universe, knock the socks off everyone on the FM band with respect to signal quality.
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