I'd also like to point out that you should use an oscilloscope to verify that a sine with a 17 kHz frequency has the same peak-to-peak voltage as one with the same digital amplitude at 7 kHz -- sound cards do have nice DACs (bitdepth-wise), but the analog filtering that you must do after DAC'ing tends to be implemented in a one-stage passive way (at least for the cheap consumer-grade sound cards I've encountered), which will not make your spectrum pretty. However, you said there's an existing Matlab implementation -- was the "Soundcard channel" measured with respect to frequency selectivity?
Best regards, Marcus On 06/03/2015 03:12 PM, Wunsch, Felix (CEL) wrote: > Hi, > > please remember to always reply to the list. > > If you mix your baseband signal in a way that no part of your spectrum > includes negative frequencies BEFORE doing the complex-to-real > conversion, this should work. At least from a signal processing > perspective. I never used my soundcard as a transmitter, so I can't > really help you with that. > > --Felix > > On 03.06.2015 15:02, [email protected] wrote: >> hi felix, >> >> >>I implemented a transmitter for the DRM(30) standard in GNU Radio >> and published it on github (https://github.com/kit-cel/gr-drm). >> >> i have already installed your transmitter and tested it with DREAM. >> it works perfect. excellent job! >> >> if i have an expert right here i can be more specific for my project: >> the aim for this work is not to implement a DRM transmitter/receiver, >> it's to implement an OFDM transmitter/receiver. previous >> implementations from my colleagues are done in Matlab. now i have to >> implement this work with an open source software (GRC) because there >> were problems with licensing and changed or removed functions in >> higher versions of Matlab. they used two computers with their sound >> cards, one for tx and one for rx. that's the reason why a center >> frequency of 12khz is used. the signal, slightly specified by the DRM >> standard (RM A, SO 3 --> bandwidth 10khz, occupied carriers 228 (-114 >> to +114)) has the highest frequency at 17khz. this signal you can >> handle with your sound card. >> >> to your comments: >> >> 1) i know there are only specific sample rates allowed for DRM but as >> written previously i will not implement the full standard. at the >> moment i use a random source block to get in some random data. >> >> 2) i don't have an USRP. i want to use a sound card to get my signal >> out to the world (at 12khz). is this possible by using a >> complex-to-float block and an audio sink? >> >> thank you very much for your help and your answers! >> >> >> alphonso >> >> *Gesendet:* Mittwoch, 03. Juni 2015 um 04:57 Uhr >> *Von:* "Wunsch, Felix (CEL)" <[email protected]> >> *An:* "[email protected]" <[email protected]>, "[email protected]" >> <[email protected]> >> *Betreff:* Re: [Discuss-gnuradio] problem with ofdm bandwidth and >> sampling rate >> Hi Alphonso, >> >> I implemented a transmitter for the DRM(30) standard in GNU Radio and >> published it on github (https://github.com/kit-cel/gr-drm). >> >> To (partly) answer your questions: >> >> 1) DRM only accepts specific audio sample rates, make sure to read >> the standard documents carefully. >> 2) Mixing is usually done by the USRP. You as the user only deal with >> the complex baseband centered around DC. 12 kHz would also be a VERY >> low carrier frequency, short-wave radio usually uses frequencies >> between 3 and 30 MHz. >> >> --Felix >> >> >> >> On 03.06.2015 12:57, [email protected] wrote: >> >> hello, >> i'm trying to implement an ofdm transmitter (and afterwards a >> receiver) for demonstration. the parameters should match the drm >> standard. so my first try was to implement the transmitter with >> the following main parameters: >> >> ofdm transmitter block >> fft length = 256 (--> fft length >= occupied carriers) >> cyclic prefix = 64 (--> 256/4) >> occupied carriers = 228 (--> (-114, -113, ..., 113, 114)) >> >> for this try no pilot carriers and symbols and no sync words. >> everything works fine but there is a problem with the spectrum of >> this ofdm signal. from older posts i have the following equation >> to set the bandwidth (bw) of my ofdm output signal: >> >> bw = sampling_rate * occupied_carriers / fft_length >> >> with the parameters above and the wish of a bandwidth of 10khz >> (to match the drm standard) i will get the following sampling rate: >> >> sampling_rate = bw * fft_length / occupied_carriers = 10khz * 256 >> / 228 = 11.228khz >> >> now i have two questions: >> >> 1) if there is an input source with a sampling rate higher than >> 11.228khz (maybe from my sound card (44.1khz) or a wav file (48 >> khz)) should i use a resampler block because there is a problem >> with nyquist? how can i handle this problem? >> >> 2) the same problem goes with the output. if i mix up the ofdm >> signal to a carrier frequency (for example 12khz) how can i >> handle this? there is also a problem with nyquist. >> >> thanks in advance! >> >> alphonso >> >> >> -- >> Karlsruhe Institute of Technology (KIT) >> Communications Engineering Lab (CEL) >> >> Felix Wunsch, M. Sc. >> Research Associate >> >> Kaiserstraße 12 >> Building 05.01 >> 76131 Karlsruhe >> >> Phone: +49 721 608-46276 >> Fax: +49 721 608-46071 >> E-Mail: [email protected] >> >> www.cel.kit.edu >> >> KIT -- University of the State of Baden-Württemberg and >> National Laboratory of the Helmholtz Association >> > > -- > Karlsruhe Institute of Technology (KIT) > Communications Engineering Lab (CEL) > > Felix Wunsch, M. Sc. > Research Associate > > Kaiserstraße 12 > Building 05.01 > 76131 Karlsruhe > > Phone: +49 721 608-46276 > Fax: +49 721 608-46071 > E-Mail: [email protected] > > www.cel.kit.edu > > KIT -- University of the State of Baden-Württemberg and > National Laboratory of the Helmholtz Association > > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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