Lesson Objectives Explain how sound can be sampled and stored in digital form Explain how sampling intervals and other considerations affect the size of a sound file and the quality of its playback.

What is sound? You should know from your Science lessons that sound is actually the subtle vibration of air which is recognised by your brain as sound.

Sound is measured in “cycles per second” or “Hertz” Sound is measured in “cycles per second” or “Hertz”. This is the frequency of the sound, or the pitch. When have you heard these terms before?

Real sounds are analogue, and we listen to them with our analogue ears Real sounds are analogue, and we listen to them with our analogue ears. However, computers can only store data in binary therefore these analogue sounds must be converted into digital files

http://www.youtube.com/watch?v=g3tfly9mKhY

Sampling This conversion from ANALOGUE to DIGITAL is called “Sampling” This takes measurements at regular intervals on the sound wave Sample No. Measurements 1 0.6 2 0.5 3 -0.3 4 -0.8 5 -0.1 6 0.8 7 The measurements are then converted into binary numbers and stored as the digital file.

Sampling turns a smooth sound wave into a boxy digital wave This means that the digital sound is not going to have the same clarity as the analogue sound

Your turn Write down the measurements for the analogue signal in the table, and recreate the file as a digital wave on the next page.

Sample Rate The difference between the analogue and digital files you have just created is quite vast. How could we make them more similar?

Sample Rate The sample rate is the rate at which the measurements are taken. The closer the measurements are taken, the smoother the digital wave will be (and the better the quality) However, if we have more measurements how will this impact the size of the file?

Bit rate/depth Another thing that effects the quality is the bitrate. The bit rate refers to the amount of bits used to store the different levels of sound at each sampling interval. The more bits, the greater range of levels that can be distinguished. This also has the affect of increasing the file size if the bit rate is high (e,g. 16-bit instead of 8-bit).

Getting the balance right 11khz – 8bit – 3.50MB – CD Quality 22khz – 8bit – 0.42MB - Basic 44khz – 16bit – 0.12MB - Telephone The 11KHz sound sample is far lower quality than the 44KHz, but on the other hand it is 30 times smaller. The balance has to be right between bitrate, sample rate and file size.

Extension On the computer open Audacity. Open Clocks by Coldplay in Common K>Computing>Binary Data Go to File Export and Options. Change the bitrate/sampling options to the minimum option. Save the file and compare the quality on play back