Cambridge A Level Information Technology (ICT): Video and audio editing (9626) – Revision Notes

For full explanations and worked examples, use the Video and audio editing study guide. These notes condense topic 11, Video and audio editing, of Cambridge International AS & A Level Information Technology (9626), following the syllabus for examination in 2025, 2026 and 2027 (version 3), sections 11.1 Video editing and 11.2 Audio editing. It is an AS Level topic. Paper 1 (Theory) questions are based on sections 1–11 and Paper 2 (Practical) tasks on sections 8–11, so revise both the reasons behind each feature and the editing skills themselves.

Links: course hub, 9626 checklist, Video and audio editing practice questions, free 10-minute diagnostics.

Topic 11 at a glance

11.1 Video editing – four strands:

1. Editing a clip for its application and audience (structure, text, effects, still images, audio, speed, filters, export, compression).
2. The effects of compression methods on video quality.
3. Why typical video editing features are used.
4. Advantages and disadvantages of export formats (MP4, AVI, MOV, WMV).

11.2 Audio editing – six strands:

1. Editing a clip for its application and audience (tracks, normalising, trimming, splicing, fades, speed, pitch, echo, reverb, stereo to mono, EQ and filters, noise reduction, overdubbing, sample rates, export).
2. How and why typical audio editing features are used.
3. Advantages and disadvantages of export formats (MP3, MP4a, WAV, AAC).
4. Why file size depends on sampling rate and sampling resolution.
5. The effects of compression on audio quality.
6. Files v containers; lossy and lossless.

Video definitions

Term	Meaning
Aspect ratio	Frame width : height, e.g. 16:9 (widescreen), 4:3, 9:16 (upright phone)
Trim	Remove unwanted footage from the start or end of a clip
Crop	Remove unwanted edges of the frame or image
Splice	Join clips end to end
Title / text-based slide	Text introducing the video or a section; a full frame of text
Subtitle	On-screen text of the spoken words
Caption	On-screen text of speech and other sounds, e.g. “[phone rings]”
Credits	Names of contributors and sources of music, footage and images used
Fade	Gradual change from or to black (or silence)
Pan / zoom	Move across / move into the frame
Transition	Effect between two clips, e.g. dissolve, wipe
Colour correction	Adjust brightness, contrast, colour balance so shots match
Still image	One frame extracted from video, or an inserted picture

Why each video feature is used

Feature	Reason (link it to the audience)
Trimming, cropping	Cut irrelevant material; shorter, focused video
Text-based slides	Introduce, separate sections, give instructions
Credits	Acknowledge people and sources; copyright and licences
Captions, subtitles	Deaf and hard-of-hearing viewers; muted viewing; other languages
Fades	Smooth, professional start and end
Extracting a still	Thumbnail, poster, freeze on a key moment
Inserting a still	Logo, diagram or photo not in the footage
Editing audio	Clear speech over music; remove distractions
Export formats	Play on the target device or platform
Compressing to resolutions	Fit storage, bandwidth and screen size

Scrolling credits: blank lines between groups (cast, crew, music, images) and before and after the list, so each group reads clearly and the text enters and leaves the screen cleanly.

Readable text: large plain font, strong contrast (or outline/shadow), few words, enough time on screen.

Method in steps – fit a still image to the video

1. Find the video’s aspect ratio (e.g. 16:9).
2. Keep one dimension of the image and work out the other: height = width × 9 ÷ 16.
3. Crop the image to that shape.
4. Resize by one scale factor applied to both width and height.
5. Check: new width ÷ new height equals the video’s ratio. No stretching.

Video export formats

Format	For	Against
MP4	Small for its quality; plays almost everywhere; web and mobile	Usually lossy
AVI	Long-established; widely played on Windows	Often large; less suited to web and mobile
MOV	Apple QuickTime; high quality; suits editing	Large; best on Apple software
WMV	Microsoft; small files	Weaker support on non-Windows devices

Compress to the medium: DVD = standard definition for DVD players; internet = resolution and bit rate that stream smoothly; mobile = small resolution and file to save storage and data.

Video compression – effects on quality

Method	File size	What the viewer notices
Lossless	Smaller than raw, still large	No change
Lossy (stronger setting)	Much smaller	Blockiness, blur, colour banding, worse in fast motion
Lower resolution	Smaller	Less sharp, especially on large screens
Lower frame rate	Smaller	Jerkier motion
Lower bit rate	Smaller	More artefacts
Interframe	Much smaller	Stores changes between key frames; suffers when the whole picture changes
Intraframe	Larger than interframe	Each frame compressed alone, so quality does not depend on other frames

Audio definitions

Term	Meaning
Normalise	Same gain applied to whole clip so the loudest peak hits a target level
DC offset	Waveform shifted off the zero line; remove it before normalising
Fade in / out	Volume rises from / falls to silence
Pitch	How high or low the sound is
Delay / echo	Distinct, quieter repeats after a set time
Reverberation	Many dense reflections: sounds like a room or hall
Stereo to mono	Two channels combined into one
Equalisation	Boost or cut chosen frequency bands
High pass filter	Keeps frequencies above cut-off; removes rumble, hum
Low pass filter	Keeps frequencies below cut-off; removes hiss
Noise reduction	Learns a noise-only section, then reduces that noise throughout
Overdub / voiceover	New recording laid over or alongside existing audio; narration over music
Sampling rate	Samples per second (Hz)
Sampling resolution	Bits per sample (bit depth)

Sampling formula

Uncompressed size (bits) = sampling rate × sampling resolution × channels × seconds
Bytes = bits ÷ 8

Doubling the rate, the resolution, the channels or the length each doubles the size. Higher rate and resolution give better quality, so the trade-off is always quality against file size.

Sample rate to suit the medium: high for music and master copies; lower for speech, voice messages and web clips.

Audio export formats

Format	For	Against
MP3	Small; plays almost everywhere	Lossy; poor at low bit rates; poor for re-editing
MP4a	MPEG-4 audio (often .m4a), usually AAC: good quality for size	Not every older player supports it
WAV	Usually uncompressed: full quality; best for editing	Very large
AAC	Lossy, generally better than MP3 at the same bit rate	Slightly less universal than MP3

Must-know distinctions

• Lossy v lossless: lossy throws data away for good (small files, some quality loss); lossless rebuilds the original exactly (no quality loss, bigger files).
• File v container: a plain file holds one stream encoded one way; a container (MP4, MOV) wraps one or more streams plus information about them. The codec inside sets the compression.
• Subtitle v caption: captions include non-speech sounds.
• Trim v crop: trim removes time; crop removes edges of the frame.
• Normalise v noise reduction: normalising changes level only; noise reduction removes unwanted sound.
• Echo v reverb: echo = distinct repeats; reverb = blended reflections.
• High pass v low pass: named after what they let through.

Quick self-test

1. State what is meant by sampling resolution.
2. Calculate the size in kB of 10 s of mono audio at 8,000 Hz and 8 bits (1 kB = 1,000 bytes).
3. A WAV file is 10 MB in stereo. State its size after converting to mono, other settings unchanged.
4. A 90 s clip is played at 200% speed. State its new length.
5. A 30 s clip is played at 50% speed. State its new length.
6. A photo is 1600 × 1200. State the size it should be cropped to, keeping the width, for a 16:9 video.
7. State the aspect ratio of a 1440 × 1080 frame.
8. Give one difference between a caption and a subtitle.
9. Explain why DC offset is removed before normalising.
10. State which filter removes low-frequency hum.
11. Give one advantage and one disadvantage of WAV.
12. Explain why a video for mobile phones is compressed to a lower resolution.

Answers

1. The number of bits used to store each sample.
2. 8,000 × 8 × 1 × 10 = 640,000 bits; ÷ 8 = 80,000 bytes = 80 kB.
3. 5 MB (half the channels, half the data).
4. 90 ÷ 2 = 45 s.
5. 30 ÷ 0.5 = 60 s.
6. Height = 1600 × 9 ÷ 16 = 900, so 1600 × 900.
7. 1440 : 1080 = 4:3.
8. Captions also describe non-speech sounds such as music or a door slamming; subtitles show speech only.
9. An offset waveform is not centred on zero, so one side’s peaks are larger. Normalising would be limited by that side and would amplify the offset too; centring first lets it set the level correctly.
10. High pass filter.
11. Advantage: full quality, ideal for editing. Disadvantage: very large files.
12. A smaller screen does not need high resolution, and smaller files save storage space and mobile data and download faster.

Where marks are usually lost

• Calling any on-screen text a “subtitle” when the question wants captions for deaf viewers.
• Giving a feature’s reason without linking it to the stated audience or medium.
• Mixing up high and low pass filters.
• Writing “normalising makes it louder”. It sets the peak to a target level, which may be quieter.
• Leaving out the ÷ 8 or the number of channels in a file-size calculation.
• Saying “lossy is worse quality” with no description of what is heard or seen.
• Stating a format advantage that is really a disadvantage elsewhere (e.g. WAV “high quality” with no mention of size) when the question asks for both.
• Resizing a still image to the video’s dimensions directly, which stretches it.
• Confusing a container with a codec: MP4 does not on its own tell you the compression used.

Official syllabus

Cambridge International AS & A Level Information Technology 9626, syllabus for examination in 2025, 2026 and 2027, version 3, Cambridge University Press & Assessment. Topic 11: Video and audio editing (11.1 Video editing; 11.2 Audio editing).