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Greetings: I have a DSK 'C31 (tools: Dsk3121) and I'm trying to make some experiences on it. I would appreciate your help with the next questions: - I connected a sin signal (3 kHz, 1.2 V) to sample it (512 points Fs=8kHz), and stored the points in a memory's sector. When I looked at the values (memd) they were so big, (153550848, 126091264, 126091263, 168057025, 74043232, 74043168, 168057027, 211564171....). I read thath these values are in two's complement format, how can I give a physical sense to these values to represent the sin signal (1.2, 1.1, 1.0 , ...)? - In the FFT1024.ASM program (included in the DSK3121), the data arrays are packed, and theres a quick log routine. How does it work? and: How can I get the real values from the specter to get the harmonics from any signal? Thanks for your help Boris Corrales _________________________________________________________ |
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FFT & sampling
Started by ●February 12, 2004
Reply by ●February 12, 20042004-02-12
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Hello Boris The FFT outputs floating point, not decimal outputs, so you should be using the 'memf' command to look at the raw data values directly. I dont recall everything about that particular FFT, but the raw data could be in bit-reverse form making it difficult to decipher. The array that goes PC is created from the raw data using the 'fastlog' routine you found. Basically the fastlog output is an 8 bit log value expressed in 7.1 fixed point allowing it to be packed into a smaller 128 word array that does go to the PC. This gives a 4:1 improvement in transfer speed to the host PC. But before you beat your head agains the wall, you might want to download the latest tools at VC33 DSK Web Page: focus.ti.com/docs/toolsw/folders/print/tmdsdsk33.html C3x DSK Code: www-s.ti.com/sc/psheets/sprc147/sprc147.zip The C31 DSK is supported, but the demo development basically stopped when the VC33 DSK came about. There are two FFT analyzers for the C31 DSK as well as a much more spiffed up Windows version for the VC33 DSK. What you will likely appreciate most is that these FFT's have a frequency auto measurement feature that scans the FFT data for the strongest signal followed by a centroiding of the full precision raw data (only needs to download 7 full precision values) to get a very accurate frequency measurement. Hint: If you dig into this part of the code (on the PC side) you should be able to determine where the harmonics you are interested will be. The fast logarithm is based on the fact that the exponent of a floating point value is a log2 value that scales a fractional value (the mantissa) by powers of 2 (..0.25, 0.5, 1, 2, 4...). The fractional mantissa provides the additional accuracy and has a value ranging from 1.0-1.99. Interestingly, when the mantissa is also take in log2 form, the fractional part almost matches the true log2 value making it possible in many cases to be used as an approximation. For additional fast logarithm information I suggest looking into the DSK3HELP.HLP file that comes with the latest tools. This is a big help file, so it will help if you do a 'Content->Find' search on 'logarithm' to find it. Best regards, Keith Larson - At 10:23 AM 2/12/04 -0600, you wrote: Greetings: I have a DSK 'C31 (tools: Dsk3121) and I'm trying to make some experiences on it. I would appreciate your help with the next questions: - I connected a sin signal (3 kHz, 1.2 V) to sample it (512 points Fs=8kHz), and stored the points in a memory's sector. When I looked at the values (memd) they were so big, (153550848, 126091264, 126091263, 168057025, 74043232, 74043168, 168057027, 211564171....). I read thath these values are in two's complement format, how can I give a physical sense to these values to represent the sin signal (1.2, 1.1, 1.0 ,...)? - In the FFT1024.ASM program (included in the DSK3121), the data arrays are packed, and theres a quick log routine. How does it work? and: How can I get the real values from the specter to get the harmonics from any signal? Thanks for your help Boris Corrales +--------------------------+ | Keith Larson | | Member Group Technical Staff | | Texas Instruments Incorporated | | | | 281-274-3288 | | | | www.micro.ti.com/~klarson (TI internal web only) | |--------------------------+ | TMS320C3x/C4x/VC33 Applications | | | | TMS320VC33 | | The lowest cost and lowest power 500 w/Mflop | | floating point DSP on the planet! | | | | Web: focus.ti.com/docs/toolsw/folders/print/tmdsdsk33.html | | Code: www-s.ti.com/sc/psheets/sprc147/sprc147.zip | +--------------------------+ |
