# GCSE Computing#BristolMet Session Objectives#10 MUST define the term sample rate COULD explain how sound can be sampled and stored in digital form SHOULD.

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GCSE Computing#BristolMet Session Objectives#10 MUST define the term sample rate COULD explain how sound can be sampled and stored in digital form SHOULD explain how sampling intervals and bit rates affect file size and playback quality Create yousrc apps to move images and play sounds.

GCSE Computing#BristolMet Binary Representation of Sound Starter: Units of storage starter Sound is usually in analogue form (continuously changing data) but the computer needs to have this in digital form as a set of binary numbers. To convert sound from analogue into a digital form, the sound wave is sampled at set intervals and the height of the sound wave (amplitude) is recorded. These values are then converted to binary and stored. The frequency of these recordings is called the sample rate. The higher the number of recording, over a given period of time, the higher the sample rate = better qual;ity recording, and larger size file.

GCSE Computing#BristolMet Make Some Noise! TASK: Use Audacity to make recordings. Export the sound files using different file types and make a note of the file size of each? Use a WAV, MP3 and one of your choice. What comparisons/differences do you notice about the different types? Prepare to feedback to the class.

GCSE Computing#BristolMet Managing Sound Files Size There are three ways of reducing the size of sound file: 1)Sample Rate: By reducing the sample rate (e.g. to 22kHz), you reduce the amount of data you need to store. This has quite serious effects on the quality of the audio 2)Sample Depth: Much as with colour, you can reduce the precision of each datum, using only 8 bits per sample instead of 24, for example. 3)Compression: Ideally you would sample at the highest practical rate you can and these use an encoding algorithm to reduce the output file size (this is why Audacity projects are much bigger in size than the files output). This can be Lossless compression (e.g. FLAC and potentially MP3/AAC/Ogg Vorbis files]) or Lossy compression (e.g. most MP3, AAC and Ogg Vorbis files). WAV and AIFF files are used for storing uncompressed audio.

GCSE Computing#BristolMet Binary Representation of Sound This is what a computer file for a music track looks like. Once again it is just binary, like our images. The metadata tells the computer what type of file it is so that it doesn’t try to display it as a picture or any other file.

GCSE Computing#BristolMet Binary Representation of Sound In order to digitally record sound, a microphone is used, a device that converts the analogue sound into a digital form by altering the output signal from the device. In order to store this digitally, the voltage is sampled at frequent intervals (typically 48 000 times per second, or 48kHz) and stored as a binary code (typically 16 or 32 bits per sample). This equates to slightly over 1.5 million bits per second, or 88 MB per minute. An 80 minute album stored in such a manner would require 6.9 GB of storage.

GCSE Computing#BristolMet Make some noise! TASK: Create some recordings in Audacity and experiment with the different sample and compression settings.

GCSE Computing#BristolMet Managing Sound Files REVISION OF SOUND & IMAGES: http://web.eecs.utk.edu/research/cs100modules/module4/

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