Stretch tasks Representing Sound

Round 1: Quiz Blitz

1. Measuring a wave's height at regular intervals Each measurement is recorded as a binary number.
2. Sample resolution It is also called bit depth, but AQA uses sample resolution, so use it.
3. 44100 CD quality is 44,100 Hz.
4. sample rate x sample resolution x duration Duration must be in seconds, and stereo doubles it.
5. False That is sample resolution. Sample rate is about how often you measure.
6. 120 Always convert to seconds before using the formula.
7. Doubles it Two channels means twice the data.
8. 65536 2 to the power 16 is 65,536.
9. 8000 Hz Speech needs far less detail than music.
10. A higher sample rate; A higher sample resolution More often and more precisely: both cost more bits.
11. False It approximates it. The difference is the error introduced by digitising.
12. The error introduced by digitising More levels mean smaller rounding, so a smaller error.

Round 2: Quick Fire

1. • Sample rate is how many samples are taken per second
   • Sample resolution is how many bits store each sample
   Model answer

   Sample rate is how many samples are taken per second, which is detail across time. Sample resolution is how many bits are used to store each sample, which is how precisely its height is recorded.

2. • Samples are taken at intervals, missing what happens in between
   • Each sample is rounded to the nearest available level
   Model answer

   Samples are only taken at intervals, so whatever the wave does in between is missed, and each sample is rounded to the nearest available level.

3. • Lower the sample rate or resolution, record in mono, or shorten it
   • The recording becomes less faithful to the original
   Model answer

   Lower the sample rate, or record in mono instead of stereo. Either makes the file smaller, but a lower sample rate makes the recording less faithful to the original.

4. • 8000 x 8 x 60 = 3,840,000 bits
   • / 8 = 480,000 bytes
   Model answer

   8000 x 8 x 60 = 3,840,000 bits, and 3,840,000 / 8 = 480,000 bytes.

Round 3: Big Think

1. • 2 minutes is 120 seconds
   • 44,100 x 16 x 120 = 84,672,000 bits per channel
   • x 2 for stereo = 169,344,000 bits
   • / 8, then / 1000 twice = 21.168 MB
   Model answer

   Duration = 2 x 60 = 120 seconds. 44,100 x 16 x 120 = 84,672,000 bits for one channel. Stereo doubles it: 169,344,000 bits. 169,344,000 / 8 = 21,168,000 bytes, which is 21,168 kB, or 21.168 MB.

2. • Sample rate is how many samples are taken per second
   • A higher rate follows the wave more closely and keeps higher frequencies
   • Sample resolution is the bits stored for each sample
   • More bits give more levels, so smaller rounding errors
   • Doubling either setting doubles the file
   • Size grows in proportion, from the size formula
   Model answer

   Sample rate is how many samples are taken per second. A higher rate gives narrower steps, so the digital version follows the original wave more closely and higher frequencies survive. Sample resolution is the number of bits used for each sample. More bits give more levels, 65,536 for 16 bits, so each height is rounded less and the recording is more faithful. But the size formula multiplies both settings together, so the file grows in proportion: doubling either one doubles the file.

This or That

1. A higher sample rate Measuring more often lets faster changes in the wave survive.
2. Hertz Samples per second, in hertz.
3. Sample resolution Resolution is how precisely each height is recorded.
4. Sound waves in the air A real sound wave has a value at every instant.
5. 2 Mono has one channel.
6. Uncompressed audio takes tens of MB A few minutes at CD quality runs to tens of megabytes.
7. 180 seconds The formula needs the duration in seconds.
8. 256 2 to the power 8 is 256.

Memory Match

• A 5 Sampling: Measuring a wave at regular intervals
• B 3 Sample rate: Measurements per second, in hertz
• C 1 Sample resolution: Bits stored per measurement
• D 6 Analogue: Has a value at every instant
• E 4 Channel: One stream of audio
• F 2 44,100 Hz: CD quality

Sort It

1. Bigger Raising the sample resolution from 8 to 16 bits Every sample needs twice the bits.
2. No change Playing it on better speakers The file is the same whatever plays it.
3. No change Playing it louder Volume on playback does not change the stored data.
4. Bigger Recording for twice as long Twice the seconds, twice the samples.
5. No change Copying it to another folder A copy holds exactly the same data.
6. Bigger Doubling the sample rate Twice as many samples to store.
7. Smaller Cutting the recording to half its length Half the duration.
8. Smaller Halving the sample rate Half as many samples.
9. Smaller Using 8-bit instead of 16-bit samples Half the bits for each sample.
10. Smaller Switching from stereo to mono One channel instead of two.
11. Bigger Recording in stereo instead of mono Two channels instead of one.
12. No change Renaming the file The samples are unchanged.