1.3 Data storage and compression

## Measurement of the Size of Computer Memories

- A binary digit is referred to as a **BIT**
- 8 bits is a **byte**
- 4 bits is a **nibble**
- Byte is used to measure memory size

**IECB System (more commonly used):**

| Name of memory size | No. of Bytes | Equivalent Denary Value |
| ------------------- | -------------- | --------------------------- |
| 1 kibibyte (1KB) | 210 | 1 024 bytes |
| 1 mibibyte (1MB) | 220 | 1 048 576 bytes |
| 1 gibibyte (1GB) | 230 | 1 073 741 824 bytes |
| 1 tibibyte (1TB) | 240 | 1 099 511 627 776 bytes |
| 1 pibibyte (1PB) | 250 | 1 125 899 906 842 624 bytes |

**Conventional System:**

| Name of memory size | No. of Bytes | Equivalent Denary Value |
| ------------------- | --------------- | --------------------------- |
| 1 kilobyte (1KB) | 103 | 1 000 bytes |
| 1 megabyte (1MB) | 106 | 1 000 000 bytes |
| 1 gigabyte (1GB) | 109 | 1 000 000 000 bytes |
| 1 terabyte (1TB) | 1012 | 1 000 000 000 000 bytes |
| 1 petabyte (1PB) | 1015 | 1 000 000 000 000 000 bytes |

### Calculation of File Size

- The file size of an image is calculated as: image resolution (in pixels) × colour depth (in bits)
- The size of a mono sound file is calculated as: sample rate (in Hz) × sample resolution (in bits) × length of sample (in seconds). `(For a stereo sound file, you would then multiply the result by two.)`

## File Types

**Musical Instrument Digital Format (MIDI)**

- Storage of music files
- A communications protocol that allows electronic musical instruments to interact with each other
- Stored as a series of demands but no actual music notes
- Uses 8-bit serial transmission (asynchronous)
- Each MIDI command has a sequence of bytes:
  - The first byte is the status byte – which informs the MIDI device what function to perform
  - Encoded in the status byte is the MIDI channel (operates on 16 different channels)
- Examples of MIDI commands:
  - Note on/off: indicates that a key has been pressed
  - Key pressure: indicates how hard it has been pressed (loudness of music)
- Needs a lot of memory storage

**MP3**

- Uses technology known as Audio Compression to convert music and other sounds into an MP3 file format
- This compression reduces the normal file size by 90%
  - Done using file compression algorithms which use Perceptual Music Shaping
  - Removes sounds that the human ear cannot hear properly
  - Certain sounds are removed without affecting the quality too much
- CD files are converted using File Compression Software
- Use lossy format as the original file is lost following the compression algorithm

**MP4**

- This format allows the storage of multimedia files rather than just sound
- Music, videos, photos and animations can be stored
- Videos could be streamed without losing any real discernible quality

**Joint Photographic Experts Group (JPEG)**

- JPEG is a file format used to reduce photographic file sizes
- Reducing picture resolution is changing the number of pixels per centimetre
- When a photographic file undergoes compression, file size is reduced
- JPEG will reduce the raw bitmap image by a factor between 5 and 15
## Lossless and Lossy File Compression

### State what is meant by data compression

- Reducing the file size
### Lossless File Compression

- All the data bits from the original file are reconstructed when the file again is uncompressed
- Important for files where the loss of data would be disastrous (spreadsheet)
- An algorithm is used to compress data
- No data is lost
- Repeated patterns/text are grouped together in indexes

### Run-Length Encoding

- It reduces the size of a string of adjacent, identical data (e.g. repeated colours in an image)
- A repeating string is encoded into two values: the first value represents the number of identical data items (e.g. characters), and the second value represents the code of the data item (such as ASCII code if it is a keyboard character), e.g. ‘aaaaabbbbccddddd’ becomes “05 97 04 98 02 99 05 100”
- RLE is only effective where there is a long run of repeated units/bits
- One difficulty is that RLE compression isn't very good for strings like "cdcdcdcdcd". We use a flag to solve this; e.g. 255 can be made the flag. Now 255 will be put before every repeating value, e.g. our previous example becomes `255 05 97 255 04 98 255 02 99 255 05 100` where 255 now indicates that the next character/set of characters is approaching

#### How lossless compression compresses the text file

- A compression algorithm is used such as RLE/run length encoding
- Repeating words/characters/phrases are identified and indexed
- … with number of occurrences
- … with their position

### Lossy File Compression

- A compression algorithm is used
- Data will be lost/deleted permanently // original file cannot be recreated

* Reduce the range of colours used / colour depth / bits per pixel
* Reduce the number of pixels / image resolution removes data that will not be noticed by the user

#### Describe how lossy compression reduces the size of the sound file

- A compression algorithm is used Discards any unnecessary sounds ...
- ... using perceptual musical shaping
- .... such as removing background noise / sounds humans can't hear // or other suitable example Reduces sample size / resolution // by example
- Reduces sample rate // by example
- Sound is clipped
- The data is permanently removed

##### Why use Compression

- To save storage space
- To make it quicker to transmit
- To make it small enough to attach to an email
- To reduce the bandwidth needed to transmit
- Reduces the size of the file 
- Takes up less storage space
- Quicker to transmit to device
- Less buffering

#### Explain why the musician would choose to use lossless compression instead of lossy compression.

- They want to be able to edit the original sound file
- They want the sound to be closest to the original recording 
- ... using lossy would reduce the sound quality
- ... using lossy will permanently remove some of the data