>but the prevailing theory is that effects would not be audible. >What I am doing is to simulate FM transmission. On the digital side (my PC) I take a .wav file and simulate the whole transmission incl. noise an rayleigh fading, which I can turn off. The signal is then transmitted via a DAQ board to an actual car radio. Right now I have turned off the noise and rayleigh fading so I can compare my signal without preemphasis and with preemphasis to the original wav file. The file with preemphasis should (nearly) sound like the original file, but it doesn't. Still, I am working with a FIR filter and I am really hoping that this was my mistake. If this should be the case, it will be a "pro" to: "the effect is audible". If not I must have made more mistakes... (Remember: the file without preemphasis sound fine, just a little dull (? I don't know the English word) as I expected)
designing a preemphasis filter
Started by ●October 4, 2007
Reply by ●October 8, 20072007-10-08
Reply by ●October 9, 20072007-10-09
>FDLS can be found here: >http://apollo.ee.columbia.edu/spm/external/tipsandtricks/files/TandT_Jan2007.zip > >GregI read your papers which were very helpful. I have not expected the filter design to be this hard when you want to approximate an analog filter. Now I know this is the case and I am more careful. As I understood you fill the algorithm with both, magnitude and phase response. Till now I always just looked for a good magnitude response and didn't care much about the phase response, which I want to change. The problem right now is that I cannot find a source for the preemphasis phase response and I am not shure if I can use the phase response of a simple RC circuit as a reference?
Reply by ●October 9, 20072007-10-09
On Oct 9, 3:23 am, "Narax" <soenke.trein...@unibw.de> wrote:> The problem right now is that I cannot find a source for the preemphasis > phase response and I am not shure if I can use the phase response of a > simple RC circuit as a reference?Yes. That is exactly what you should use. You might find that FDLS has difficulty matching the prototype in the uppermost half-octave. So if you're only sampling at 44100, you might get a good match up to 15 kHz or so, but not much beyond that. If you want to match all the way up to 20 kHz, upsample first. Greg
Reply by ●October 9, 20072007-10-09
Narax wrote:>> FDLS can be found here: >> http://apollo.ee.columbia.edu/spm/external/tipsandtricks/files/TandT_Jan2007.zip >> >> Greg > > I read your papers which were very helpful. I have not expected the filter > design to be this hard when you want to approximate an analog filter. Now I > know this is the case and I am more careful. > As I understood you fill the algorithm with both, magnitude and phase > response. Till now I always just looked for a good magnitude response and > didn't care much about the phase response, which I want to change. > The problem right now is that I cannot find a source for the preemphasis > phase response and I am not shure if I can use the phase response of a > simple RC circuit as a reference?The standard preemphasis response *1s* a simple RC. Linear phase won't sound terrible, but the closer to minimum phase, the better. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●October 9, 20072007-10-09
Greg Berchin wrote:> On Oct 9, 3:23 am, "Narax" <soenke.trein...@unibw.de> wrote: > >> The problem right now is that I cannot find a source for the preemphasis >> phase response and I am not shure if I can use the phase response of a >> simple RC circuit as a reference? > > Yes. That is exactly what you should use. > > You might find that FDLS has difficulty matching the prototype in the > uppermost half-octave. So if you're only sampling at 44100, you might > get a good match up to 15 kHz or so, but not much beyond that. If you > want to match all the way up to 20 kHz, upsample first.IIRC, on-air FM is only specified up to 15 KHz. Especially with preemphasis, the modulation index gets too high above that. There were few sources that could reach that high when the standard was first laid down. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●October 9, 20072007-10-09
On Oct 8, 10:22 am, "Narax" <soenke.trein...@unibw.de> wrote:> >but the prevailing theory is that effects would not be audible. > > What I am doing is to simulate FM transmission. On the digital side (my > PC) I take a .wav file and simulate the whole transmission incl. noise an > rayleigh fading, which I can turn off. The signal is then transmitted via > a DAQ board to an actual car radio. > Right now I have turned off the noise and rayleigh fading so I can compare > my signal without preemphasis and with preemphasis to the original wav > file. The file with preemphasis should (nearly) sound like the original > file, but it doesn't. Still, I am working with a FIR filter and I am > really hoping that this was my mistake. If this should be the case, it > will be a "pro" to: "the effect is audible". If not I must have made more > mistakes... (Remember: the file without preemphasis sound fine, just a > little dull (? I don't know the English word) as I expected)Notice that the pre-emphasis has about 15 dB or more boost at high frequencies, make sure that this does not casue OVEVERMODULATION to your modulator. Mark
Reply by ●October 10, 20072007-10-10
>IIRC, on-air FM is only specified up to 15 KHz. Especially with >preemphasis, the modulation index gets too high above that. There were >few sources that could reach that high when the standard was first laid >down. > >JerryThat is the case. So I don't have to worry about the last 5kHz. I have to cut above 15kHz anyway, because I need to secure the pilot tone at 19kHz which indicates a stereo transmission. For this reason there has to be a lowpass and I think I forgot about its phase response. I should be minimal phase as well when I correct?>Notice that the pre-emphasis has about 15 dB or more boost at high >frequencies, make sure that this does not casue OVERMODULATION to >your modulator. > >Mark
Reply by ●October 10, 20072007-10-10
Sorry, I accidentally hit the "Send Message!" button.>Notice that the pre-emphasis has about 15 dB or more boost at high >frequencies, make sure that this does not casue OVEVERMODULATION to >your modulator. > >MarkThank you for this hint - I didn't take this in concideration till now and I think this is the first thing the check out. One more question about the RC-curcuit: You said that I should take its phase response. But a simple RC-curcuit has a diffrent magnitude response than I want to achieve. Is a mix of the desired magnitude response and the RC-circuit's phase response a good design?
Reply by ●October 10, 20072007-10-10
On Oct 10, 2:31 am, "Narax" <soenke.trein...@unibw.de> wrote:> But a simple RC-curcuit has a diffrent magnitude response > than I want to achieve. Is a mix of the desired magnitude response and the > RC-circuit's phase response a good design?Not just "any" RC circuit; use the magnitude and phase responses from the RC circuit that implements the preemphasis filter that you want. Greg
Reply by ●October 10, 20072007-10-10
Narax wrote:>> IIRC, on-air FM is only specified up to 15 KHz. Especially with >> preemphasis, the modulation index gets too high above that. There were >> few sources that could reach that high when the standard was first laid >> down. >> >> Jerry > > That is the case. So I don't have to worry about the last 5kHz. I have to > cut above 15kHz anyway, because I need to secure the pilot tone at 19kHz > which indicates a stereo transmission. For this reason there has to be a > lowpass and I think I forgot about its phase response. I should be minimal > phase as well when I correct?You want the pilot-carrier blocking filter's influence on the phase of the passband to be as small as possible. A symmetric FIR is best. Remember that stereo is backwardly compatible with the older mono FM. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
