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I keep getting overshoots with BBP.
I measure with a pira Broadcast Analyzer http://pira.cz/fm_broadcast_analyzer and get overshoots over 80 Khz. I’ve also got a modulation meter on the transmitter where i can see that its going beyond the maximum deviation. The transmitter is a R.V.R. PTX30 UHT.
I’ve calibrated the soundcard with tilt. I’ve tried 30 Hz or 60 Hz HPF. I’ve tried a ESI Juli@ and a onboard Realtek ALC885. I’ve tried other transmitters but its the same story. I calibrated the transmitter to 75 KHz deviation with the 400 Hz tone from BBP.
I’ve seen that you also use the pira broadcast analyzer, what are your experiences with it when you measure breakaway?My experience is that the pira broadcast analyzer is accurate enough to sort of set the modulation, but not to verify overshoots. In fact, if you play a sine wave straight into a transmitter at 75 kHz swing, the meter will easily bounce around between 70 and 80, maybe even beyond.
///Leif
I’ve tried to replicate it, i’ve put the 400 Hz tone tone again on the transmitter. The MAX values are going from 75.2 till 76.3 over a period of a few minutes. So that’s within the +/- 1 Khz deviation error. AVE goes from 74.9 till 75.1. My experience is that the analyzer needs a strong signal to measure good.
When i change to music MAX goes till 91.1 KHz with a clipped song but generaly above 75 Khz and with a lot of music around 80 KHz. I think this is a big difference. I also see it on my modulation meter on the transmitter who is also going till about 90 Khz with 30 Hz HPF in BBP. When i change it to 60 Hz the MAX goes till 83,5 KHz on both meters. The deviation error for typical content is +/- 2Khz for the pira analyzer so 77 Khz should still be fine.
How do you verify that there are no overshoots?
Try making sure you have BBP set to the optimal setting on the CPU, which is Maximum. The CPU settings do carry a risk of overshoots if set to lower settings. The usual reason for unexplained overshoots is still poor tilt calibration. I notice you say you have set the tilt but did you do it using a tilt figure given on the forum for your soundcard, or did you use the calibration utility to calibrate?
BBP is set on maximum CPU automatically. I’ve calibrated for tilt with the scope method and lowest deviation on the analyzer and deviation meter. The measurements i have shown here where done with my onboard soundcard. I have seen better results by someone else with the same song. I will do some testing this week or next week with the juli@ card and will post my results here. I need to order some memory to make my other system going again.
Actually, even the lowest cpu mode should never overshoot more 2%.
It seems your pira.cz analyzer works better than mine!
One thing, I can guarantee: The clipper itself doesn’t overshoot. The overshoots happen later in the chain, either in the sound card (improper reconstruction filter such as in VIA VT1708B, or due to improper frequency response) or in the transmitter (improper low frequency response is not that uncommon!).
What on-board sound card did you use, and what tilt settings did you end up using?
Also, since you have an oscilloscope, try switching BBP to mono mode (and make sure the oscilloscope is DC coupled, not AC) and watch the composite waveform carefully when playing music. It’s supposed to be extremely tightly peak controlled — there should be NO visible overshoots whatsoever, and the waveform should NOT move around. Essentially, it should be as if two invisible immobile lines were drawn on the oscilloscope which the signal never ever crosses.
If you verify this on the scope, but still see overshoots on the Pira analyzer, then it’s either the transmitter (likely poor low frequency response) or the Pira analyzer. The only way to figure out which would be to compare to a known good modulation monitor such as a Belar Wizard, preferably with a scope hooked up to its composite output.
When I’ve tested calibrated BBP with a Juli@ (or certain realtek onboard chips) into an BW PLL+ 1W exciter, received by a Belar Wizard connected to an analog oscilloscope, it is rock solid. In fact, the oscilloscope looks basically identical to BBP’s internal oscillosocpe.
///Leif
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