Compression and file properties: R093 revision

Smaller files take less storage, upload and download faster, stream without buffering, use less of a phone's data allowance, fit within email and platform limits, and make web pages load quickly. A slow page or a video that keeps buffering loses its audience, so compression is part of designing for a platform.

Lossy compression discards detail. In a photo, similar colours are merged; in audio, sounds people are unlikely to hear are dropped. The file can shrink dramatically, but the lost data is gone for good, and compressing too far causes artefacts: blocky patches, blurring and colour banding in images, or a thin, tinny sound in audio. Re-saving a lossy file again and again makes it worse. Lossy suits finished products for websites, streaming and social media, where small files matter more than perfect quality.

Lossless compression finds patterns and stores them more efficiently, for example recording 'twenty blue pixels' instead of listing each one, so nothing is lost and the original comes back exactly. Files shrink less. Lossless suits logos and graphics with sharp edges and text, master copies, archives, anything that will be edited again, and professional print.

Pixel dimensions are the width and height of an image in pixels, such as 1920 × 1080. More pixels means more detail and a bigger file. PPI (pixels per inch) and DPI (dots per inch, used for printing) describe how densely those pixels are packed when the image is displayed or printed.

Print needs far more pixels than a screen: about 300 DPI at the printed size for sharp results. So an image 3,000 pixels wide prints sharply at about 10 inches wide; stretch it across a poster and it looks soft and pixelated. A web image only needs enough pixels to fill the space it appears in, and anything more just slows the page down.

Digital audio measures the sound wave many times a second. The sample rate is how many measurements are taken each second, in hertz: CD audio uses 44.1 kHz and video usually 48 kHz. The bit depth is how many bits store each measurement: 16-bit for CD, 24-bit in studios. More bits allow more volume levels, so quiet and loud sounds are captured more accurately. Higher sample rates and bit depths give better quality and bigger files. Stereo (two channels) also doubles the data of mono, which is why a speech-only podcast is often exported in mono.

Frame rate is the number of still frames shown each second (fps). Cinema traditionally uses 24 fps, UK television 25, and online video often 30 or 60. Higher frame rates make fast motion smoother, which suits sport and games, and allow slow motion when played back slower, but they create bigger files. Too low a frame rate makes movement look jerky.

Resolution is the frame size in pixels. SD (standard definition) is 720 × 576 in the UK; HD is 1280 × 720, with Full HD at 1920 × 1080; UHD is 3840 × 2160, often sold as 4K (cinema 4K is 4096 × 2160); 8K is 7680 × 4320. Each step up shows more detail and multiplies the file size. A video's size grows with its resolution, frame rate and length, and the bitrate it is compressed to brings it back down.

Exam scenarios often give a file that is too big for its platform. Say a bakery's finished video is 800 MB and needs to go on its website. Exporting it as MP4 with lossy compression at a lower bitrate, and at 1080p rather than 4K, shrinks it enough to load quickly for visitors on phones. The trade-off is quality: compress too far and the warm, golden dawn footage turns blocky, which undermines the hand-crafted feel the client wanted. The answer is to compress the copy for the web while keeping the uncompressed master for any future version.

Compression on its own was worth 2 marks on one of the four papers we analysed and none on the others, but file properties came up more often: January 2025 asked candidates to explain audio bit depth, and questions on resolution, frame rate and choosing formats appear in Section B scenarios.

For how the long answers and pre-production documents are marked, see how R093 is marked.