Re: [Discuss-gnuradio] FM pre-emphasis and de-emphasis
On 10/01/18 18:53, Andy Walls wrote: [snip] >> When working correctly, the amplitude/frequency response should be >> within, ideally, 0.1 dB of the values shown. I've shown values for >> both 75 and 50 microsecond time constants. >> >> Freq (Hz) Gain dB (75us) Gain dB (50us) >> >> 30 0.000.00 >> 300 -0.09 -0.04 >> 500 -0.23 -0.11 >> 1000 -0.87 -0.41 >> 3000 -4.77 -2.76 >> 5000 -8.16 -5.40 >> 8000 -11.82 -8.64 >> 1-13.65 -10.36 >> 12500-15.52 -12.15 >> 15000-17.07 -13.65 >> > > FWIW, the attached plots compare the 50 us de-emphasis filter response > of GNURadio 3.7.10 vs. the numbers you provided. FWIW they are not my numbers, but appeared to come from a reliable source. > The filters stay within 0.1 dB of each other up through 5000 Hz, but by > then your tabular response has noticeably started to bend upward. The > difference between the two filters is 3.44 dB at 15000 Hz. It bends up because your plot has got one log axis and one linear one. If plotted with a log frequency axis rather than a linear one then the slope at the right should tend to a straight line of 6.02 dB/octave or 20 dB/decade. Indeed the idea of dB/octave or dB/decade only makes sense if you use a log frequency axis. Alternatively, you could use a linear voltage axis instead of dB, and a linear frequency axis. That should also tend towards a straight line on the right hand side. -- Richard Lamont ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] FM pre-emphasis and de-emphasis
On 09/01/18 01:10, Andy Walls wrote: >> From: Richard Lamont >> The FM pre-emphasis and de-emphasis blocks do not appear to implement >> the amplitude-frequency response I would expect - equivalent to a >> simple analog RC filter. >> >> Is this a known bug? (version 3.7.9) > > Yes. > > I implemented the pre-emphasis filter in 3.7.10 and reworked the de- > emphasis filter at the same time. Excellent. That works. Thank you. -- Richard Lamont ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] FM pre-emphasis and de-emphasis
On 08/01/18 21:59, Müller, Marcus (CEL) wrote: > please don't reply to existing emails on the Mailing List to start a > new discussion thread Sorry about that. > Anyway: If I remember correctly, they work pretty OK – but it's hard to > mimic analog filters in digital domain, so OK might be different for > people that care more about the actual acoustic properties of the > transmission than me! Fair enough. I've just started playing with grc and did the usual FM broadcast RX thing as my Hello World - and, as a former broadcast engineer with pretty good ears - I immediately noticed that there was far too much 'top' in the the audio. I have been using the FM de-emphasis block that is shipped in the Ubuntu gnuradio 3.7.9 package - I have added any others yet. > So, I'm pretty sure you're not being thick – it's pretty possible that > GNU Radio contains a mediocre implementation. Have you seen the source > code? If you want to, we can guide you through that, and if you have > recommendations, we'll be happy to cooperate with you to implement a > better de/emph. I haven't seen the source code and wouldn't be able to do anything with it if I had. I'm an analog guy and have quite enough trouble just pointing and clicking in grc. I can't code and the maths required to calculate filter coefficients would make my head explode. That's why I'm using grc in the first place! If the block shipped with vanilla gnuradio isn't doing its job, then surely it would be best to fix that bug. When working correctly, the amplitude/frequency response should be within, ideally, 0.1 dB of the values shown. I've shown values for both 75 and 50 microsecond time constants. Freq (Hz) Gain dB (75us) Gain dB (50us) 30 0.000.00 300 -0.09 -0.04 500 -0.23 -0.11 1000 -0.87 -0.41 3000 -4.77 -2.76 5000 -8.16 -5.40 8000 -11.82 -8.64 1-13.65 -10.36 12500-15.52 -12.15 15000-17.07 -13.65 For pre-emphasis, obviously the gains should be positive. This .grc demonstrates the problem - the amplitude/frequency response is essentially flat: http://www.lamont.me.uk/misc/deemph_test.grc -- Richard Lamont ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] FM pre-emphasis and de-emphasis
I'm new to this, so my apologies if I'm being thick. I have spent hours trying to resolve this. The FM pre-emphasis and de-emphasis blocks do not appear to implement the amplitude-frequency response I would expect - equivalent to a simple analog RC filter. Is this a known bug? (version 3.7.9) TIA. -- Richard Lamont ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
