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Sampled Audio

Sampled Audio. Sampled Audio. We store digitised audio samples. This is the main audio data. In a file (e.g. .wav format) we also need some header information. We will look at the headers and data. Then we will modify the audio data. Sampled Audio.

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Sampled Audio

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  1. Sampled Audio

  2. Sampled Audio • We store digitised audio samples. • This is the main audio data. • In a file (e.g. .wav format) we also need some header information. • We will look at the headers and data. • Then we will modify the audio data.

  3. Sampled Audio • As described before, the audio signal is sampled at some sampling rate fs. • If we sample at fs samples per second how many samples will we need to represent 2 seconds of audio? • What if it is stereo?

  4. Sampled Audio • Also, we must decide on some level of quantisation or bits per sample nbits. • Total bit rate is therefore • nbits x fs bits per second. • After deciding on some coding scheme (audio signal is biplolar) we store the numeric representation of the samples sequentially.

  5. The “wav” format • Found in Microsoft environments. • Consists of a “RIFF“ header. • Broken into chunks and subchunks. • Chunks (and subchunks) have: • label of chunk • Size of chunk • Then the chunk itself (body) “wav” file always has • RIFF chunk • Fmt subchunk • Data subchunk http://www.sonicspot.com/guide/wavefiles.html

  6. The “RIFF” header • Byte 1 - 4 “RIFF” • Type of file in ASCII • Byte 5 - 8 00 01 7A F0 9700810 • Size of file from byte 9 onwards • Byte 9 – 12 “WAVE” • Form of data in ASCII • Byte 13 - 16 “fmt “ • Format in ASCII (note the space after fmt) • Byte 17 - 20 00 00 00 10 1610 • Size in bytes of the format information • Byte 21 - 22 00 01 110 • Code (1 represents Microsoft PCM)

  7. The “RIFF” header • Byte 23 – 24 00 02 210 • Number of channels • Byte 25 – 28 00 00 56 22 2205010 • Samples per second • Byte 29 – 32 00 01 58 88 8820010 • Average bytes per second • Byte 33 – 34 00 04 410 • Block alignment • Byte 35 – 36 00 10 1610 • Bits per sample • Byte 37 - 40“data” • Indicates the start of the data block in ASCII • Byte 41– 44 00 01 7A 9C 9692410 • Size in bytes of the data

  8. Block Alignmentin this example

  9. Format codes for audio data • 0x0001 • Microsoft pulse-code-modulation (PCM) format • 0x0101 • IBM m-law format • 0x0102 • IBM a-law format • 0x0103 • IBM AVC adaptive differential PCM format

  10. Other possible “subchunks” • Fact chunk • This gives information about the file which is particularly useful for other formats, for example where the data is compressed. In PCM this is not required. • Cue chunk • The cue chunk identifies points in the data stream. • Playlist chunk • Gives the order in which the data is to be played using the cue points. • Associated data chunks • Other information that you might want stored such as text.

  11. Examination in Winhex • Look at “chord.wav” in Winhex. • Find the start and end of the: • Riff chunk • Fmt subchunk • Data subchunk • Why does the end of the Riff chunk not correspond to the end of the data subchunk? • More subchunks?

  12. Examination in Winhex • Determine the sampling frequency. • Change it. • Estimate how long the sound will play for. • Double that time using Winhex. • How many bits are used per sample? (nbits)

  13. Sound file handling using Matlab • Use wavread to retrieve audio data. • snd=wavread(‘chord.wav’) • Or more properly • [snd , fs, bits]=wavread(‘chord.wav’) • Are the number of bytes returned as expected? • What is the sampling rate and the number of bits per sample.

  14. Sound file handling using Matlab • Use wavwrite to save audio data. • wavwrite(snd, ‘chord.wav’) (simplest) • See Matlab help for more options • Use sound(snd) to play sound. • More properly sound(snd,fs)

  15. Some manipulation • Cut a sound • Acut=A(start:end,:) • Join two sounds together • Newsound=[A(start:end,:) ,B(start:end,:)]

  16. Matlab Exercises • Change the sampling frequency. • Listen to the effect and examine in Winhex. • Remove part of the sound using the Matlab : operator. • Play one channel of the stereo sound using array indexing.

  17. Matlab Exercises • Change the volume by multiplication/division. • Mix two sounds together. (Must be of the same length) • Add echo to the sound, by starting a lower level version a bit later. • Invent your own effects.

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