Data representation – Sound.

Learning Objectives - Candidates should be able to: (n) explain how sound can be sampled and stored in digital form (o) explain how sampling intervals and other considerations affect the size of a sound file and quality of its playback.

Recap How many colours could be represented per pixel in a 4 bit colour pallet The more bits per pixel the greater the colour depth, better image quality Resolution is the number of pixels per inch (PPI) or (DPI) As we know, a bitmapped image when enlarge does not gain more pixel, these pixels are merely stretched

Definitions Type Definition Other Fact Analogue Continuously variable Music Speech Activation Sensor Digital Binary pattern Can be stored Can be processed

When To Use Which? Analogue Playing Music Telephone Call Analogue Music stored on USB Key Fibre optic transmission A song being edited in Audacity Electricity along a copper wire Analogue Digital Digital Digital Analogue

How do we store sound on computers? Sample the sound at set intervals. Max possible Value Max possible Value +128 +128 time time -127 -127 Min Possible Value Min Possible Value

Analogue to Digital Conversions

Sampling Sampling Resolution = The number of bits needed to store one sound sample Sampling Rate = The frequency at which samples are taken The bigger the sample interval the lower the sample rate The height of the wave is sampled at regular intervals The height is represented by a code (011, 111, …)

What affects the quality / file size? How often you take a sample…(sample rate) 44100 Hz 16000 Hz… The Sample Resolution How many bits you use for each sample 16 bits per sample gives 65,536 possible levels. (216)

Digital Mono Storage Requirement 16 bits per sample @ 44.1KHz 44100 sample X 2 / 1024 44100 Samples Sec 16 Bits Sample / 1 Byte 8 Bits / 1 Kb 1024 Bytes * How Much Storage For A 5 Second Clip? = 86.1 Kb/Sec 86.1 Kb/Sec * 5 Sec = 1033.2 Kb

Digital Stereo Storage Requirement 16 bits per sample @ 44.1KHz by 2 channels 44100 sample X 4 / 1024 1 Byte 8 Bits 1 Kb 1024 Bytes 44100 Samples Sec 16 Bits * 2 Sample * * * = 172.27 Kb/Sec How Much Storage For A 15 Second Clip? 172.27 Kb/Sec * 15 Sec = 2584.05 Kb

Limericks… There was an old person from Fratton Who would go to church with his hat on. 'If I wake up,' he said, 'With a hat on my head, I will know that it hasn't been sat on.' There was a young man from Dealing Who caught the bus for Ealing. It said on the door Don't spit on the floor So he jumped up and spat on the ceiling There was an old man from Milan, Whose limericks never would scan. When told this was so, He said, 'yes, I know. 'But I always try to get as many syllables into the last line as I possibly can.' There was an Old Man from Nantucket Who kept all his cash in a bucket. His daughter, called Nan, Ran away with a man, And as for the bucket, Nantucket

Activity… Understanding the structure of a Limerick Write your own to explain the file size of audio recordings. You need to include the words sample rate and Sample frequency

Open Audactiy Plug headphones / mic in before you open application. Once you have a Limerick Using a Microphone provided record your Limerick Start with the sample rate set to 44,100 Export the file Repeat the exercise with the sample rate set to 16,000 Explain the difference in file size Show your working.