11 Video and Audio Editing (Cambridge AS/A‑Level IT – Topics 11.1 & 11.2)
Learning Objectives
By the end of this lesson students will be able to:
- Import, organise and set project parameters for both video and audio files.
- Trim, splice, and join video and audio clips accurately, applying appropriate fades.
- Add titles, subtitles, transitions, and basic colour‑correction to video.
- Synchronise a voice‑over or soundtrack with video footage.
- Normalise, compress, apply noise‑reduction, and perform overdubbing on audio.
- Convert between mono and stereo, checking for phase‑cancellation.
- Export the final product in the required format, bitrate and resolution.
1. Project Setup & File Management
- Supported video formats: MP4 (H.264), AVI, WMV, MOV.
- Supported audio formats: WAV (uncompressed), AIFF, MP3, AAC, OGG.
- Metadata to record: file name, sample rate, bit depth, frame‑rate, aspect ratio, source device.
- Typical project settings:
| Media type | Sample rate / Bit depth | Frame‑rate | Resolution / Aspect ratio |
|---|
| Audio (speech) | 44.1 kHz / 16‑bit | — | — |
| Video (online) | — | 30 fps | 1920 × 1080 (16:9) |
2. Video Editing
2.1 Importing & Timeline Organisation
- Open the DAW/Video‑editing software (e.g., Adobe Premiere, DaVinci Resolve, or free alternatives).
- Set the project frame‑rate and resolution to match the source footage.
- Drag‑and‑drop video clips onto the video tracks; audio tracks appear automatically on the timeline.
- Rename each track (e.g., “Intro‑Video”, “Interview‑Audio”) for clarity.
2.2 Trimming & Splicing Video
- Select the clip, move the playhead to the desired start point and press I (set In). Move to the end point and press O (set Out).
- Use ripple‑delete to close the gap automatically, or manually drag the following clip leftwards.
- For a seamless splice, zoom to ≥ 200 % and apply a cross‑fade (video dissolve) of 10–20 ms.
2.3 Adding Titles, Subtitles & Lower‑Thirds
- Insert a Text layer, choose a readable font (e.g., Arial 12 pt) and set a background opacity ≤ 30 % for legibility.
- For subtitles, use the Captions panel and ensure the timing matches the spoken audio (± 0.1 s tolerance).
- Lower‑thirds are placed on a separate track, typically 2–3 s long, and can be animated with a simple slide‑in transition.
2.4 Basic Video Effects
| Effect | Typical Use | Key Parameters |
|---|
| Cut/Transition |
Join two clips; create visual interest. |
Duration (ms), type (Dissolve, Wipe, Fade‑to‑Black). |
| Pan‑and‑Zoom (Ken Burns) |
Emphasise still images or focus within a shot. |
Start/End position, speed, easing curve. |
| Colour‑Correction |
Balance exposure, adjust white‑balance. |
Lift, Gamma, Gain; Saturation; LUT (if required). |
| Video Compression |
Reduce file size for export. |
Codec (H.264/H.265), bitrate (Mbps), key‑frame interval. |
2.5 Export Settings
- Container: MP4 (H.264) for most online platforms.
- Resolution: retain original (e.g., 1920 × 1080) or down‑scale to 1280 × 720 for faster streaming.
- Audio: 44.1 kHz, 16‑bit PCM or AAC‑LC at 128 kbps.
- Check the export preset against the examination board’s specification (e.g., “1080p @ 30 fps, 5 Mbps”).
3. Audio Editing
3.1 Importing & Basic Settings
- File → Import (or drag‑and‑drop) the audio file into a new audio track.
- If prompted, confirm the sample rate (44.1 kHz) and bit depth (16‑bit).
- Rename the track (e.g., “Narration‑Raw”).
3.2 Noise‑Reduction & Over‑dubbing
- Noise‑reduction: Use a “Noise‑Gate” or “Spectral De‑noise” plugin.
- Capture a silent portion → set as noise profile.
- Apply reduction of 12–18 dB; listen for artefacts.
- Over‑dubbing (recording additional dialogue):
- Arm a new track, enable “Punch‑in/out” to record only the needed section.
- Align the new recording with the original waveform before processing.
3.3 Trimming, Splicing & Fades (Audio)
- Select unwanted region → Delete (non‑destructive) or Cut (destructive).
- Close gaps with ripple‑delete or manual slide.
- Apply a short fade‑in/out (5–10 ms for speech, 50–200 ms for music) to avoid clicks.
- When joining two clips, zoom in and add a cross‑fade of 10–20 ms.
3.4 Normalising
Normalising adjusts the overall gain so that a chosen reference level is reached. Two approaches are required by the syllabus.
3.4.1 Peak Normalising
- Target levels: –1 dBFS (broadcast), –3 dBFS (streaming), 0 dBFS (mastering, followed by a limiter).
- Formula (brief reminder):
Peak_dBFS = 20 log₁₀(A_peak / FullScale)
where A_peak is the maximum sample amplitude.
- Gain change:
ΔG = Target_dB – Peak_dBFS.
3.4.2 RMS (Loudness) Normalising
- Targets: –23 LUFS (EBU R128 broadcast) or –16 LUFS (podcasts).
- Most DAWs have a “Normalize to LUFS/RMS” command that calculates the required gain automatically.
3.4.3 Example – Peak Normalising
| Original Peak (dBFS) | Target (dBFS) | Gain Change (ΔG) | Resulting Peak (dBFS) |
|---|
| ‑6.2 | ‑1.0 | +5.2 dB | ‑1.0 |
| ‑12.5 | ‑3.0 | +9.5 dB | ‑3.0 |
3.5 Compression (Dynamic‑Range Control)
Key Parameters
- Threshold (T) – level above which gain reduction starts (dBFS).
- Ratio (R) – amount of reduction (e.g., 4:1).
- Attack (A) – time to engage after crossing T (ms).
- Release (R) – time to stop after falling below T (ms).
- Knee – hard vs. soft transition.
- Make‑up gain – restores overall level.
Compression Formula (for levels above the threshold)
L_out = T + (L_in – T) / Ratio
Practical Workflow
- Play the clip and note L_max (peak) and L_rms (quietest RMS).
- Set Threshold ≈ 3 dB below L_max.
- Choose a Ratio:
- 2:1 – gentle smoothing (music).
- 4:1 – typical for spoken word.
- 10:1 or higher – limiting for broadcast.
- Adjust Attack (≤ 10 ms for speech) and Release (100–200 ms) to avoid “pumping”.
- Apply Make‑up gain** to reach the target normalised level.
- Listen for artefacts; fine‑tune parameters as needed.
Example Parameter Table
| Material | Threshold (dBFS) | Ratio | Attack (ms) | Release (ms) | Make‑up Gain (dB) |
|---|
| Podcast interview | ‑12 | 4:1 | 10 | 150 | +3 |
| Acoustic guitar | ‑18 | 2:1 | 30 | 200 | +2 |
3.6 Additional Audio Effects
| Effect | Typical Use | Key Parameters |
|---|
| Delay / Echo |
Rhythmic repeats or ambience. |
Delay time (ms), feedback (%), mix. |
| Reverb |
Simulate room/hall; add depth. |
Room size, decay (RT60), pre‑delay, mix. |
| Pitch‑shift / Time‑stretch |
Change pitch without affecting length, or vice‑versa. |
Semitone shift, stretch factor, algorithm quality. |
3.7 Channel Conversion & Phase‑Cancellation Check
- Stereo → Mono: Use “Mix to Mono” (L+R)/2. After conversion, zoom the waveform; if large cancellations appear (silence where audio should exist), invert the polarity of one channel before summing.
- Mono → Stereo: Duplicate the mono track, pan left/right, optionally add a 5–10 ms delay to one side for a subtle stereo width.
4. Synchronising Audio with Video
- Import both the video clip and the separate audio (e.g., voice‑over).
- Set the project frame‑rate to match the video (commonly 25 fps or 30 fps).
- Align the audio waveform with a visual cue (clapperboard, lip‑movement) using the timeline’s time‑code or by dragging until the peaks match.
- Fine‑tune by nudging the audio in 1‑frame increments.
- Lock the linked audio‑video pair to prevent accidental desynchronisation.
- Export as a combined file (see Section 2.5 for video export settings).
5. Integrated Editing Example (30‑second Podcast Intro with Video)
- Import a 30‑s video (MP4, 1080p @ 30 fps) and a separate narration WAV (44.1 kHz, 16‑bit).
- Trim video: remove 0.5 s of black at start and 0.7 s of fade‑out at end.
- Trim audio: delete leading 0.8 s and trailing 1.2 s of silence; apply 5 ms fade‑in, 10 ms fade‑out.
- Noise‑reduce the narration using a captured silent segment (≈ 15 dB reduction).
- Normalise (Peak) the narration to –1 dBFS.
- Compress narration:
- Threshold = –10 dBFS
- Ratio = 5:1
- Attack = 8 ms
- Release = 120 ms
- Make‑up gain = +2 dB
- Sync the processed narration to the video timeline, aligning the first spoken word with the visual cue.
- Add a lower‑third title (“Episode 1 – Welcome”) that appears 1 s after the start and fades out after 4 s.
- Apply a short room reverb** to the narration (decay = 0.2 s, mix = 12 %).
- Export as MP4 (H.264, 5 Mbps) with embedded 44.1 kHz PCM audio.
6. Summary Checklist
- Set project sample rate, bit depth, frame‑rate, and aspect ratio before importing.
- Confirm file format and metadata meet syllabus requirements.
- Trim unwanted sections; use ripple‑delete or manual slide.
- Apply appropriate fades (audio & video) at every edit point.
- For audio: perform noise‑reduction, overdubbing, then normalise (peak or RMS).
- Compress with a threshold just below the loudest peaks; choose ratio, attack, release, and make‑up gain accordingly.
- Check mono conversion for phase cancellation; use polarity inversion if needed.
- Synchronise audio and video precisely using waveform or visual cues.
- Add titles, subtitles, transitions, and colour‑correction as required.
- Export using the correct container, codec, bitrate, and audio settings.
7. Assessment‑Style Questions
- Explain why normalising does not change the dynamic range of a recording.
- A 10‑second clip has a peak level of –8 dBFS. What gain must be applied to normalise it to –1 dBFS?
- Describe the audible effect of using a very fast attack time on a vocal track.
- Given a threshold of –12 dBFS and a ratio of 4:1, calculate the output level for an input of –6 dBFS.
- What is the difference between a ripple‑delete trim and a non‑destructive trim?
- When would you choose RMS normalising over peak normalising?
- Outline the steps required to convert a stereo interview recording to mono without introducing phase cancellation.
- List three video‑specific editing actions required by the Cambridge syllabus and give a brief purpose for each.
- Explain how you would synchronise a separately recorded voice‑over to a video clip.
8. Practice Tasks (Exam‑Style)
- Video‑only task: Using a supplied 15‑second video clip, trim the start and end, add a 2‑second fade‑in, a 2‑second fade‑out, insert a lower‑third title, apply a dissolve transition to a second clip, and export the result as an MP4 (1080p, 5 Mbps).
- Audio‑only task: Given a 20‑second WAV recording of a speech, remove background hiss with noise‑reduction, trim silence, normalise to –23 LUFS, compress with a 4:1 ratio, and export as a 44.1 kHz, 16‑bit WAV.
- Integrated task: Combine a 10‑second video of a product demo with a separately recorded voice‑over. Align the audio to the video, add a subtitle track, apply a short room reverb to the voice‑over, convert the final mix to stereo, and export as MP4 (720p, 30 fps).
9. Suggested Diagram
Flowchart (to be drawn on the board or hand‑out):
- Import → Trim → Fade‑in/out → Noise‑reduce/Over‑dub → Normalise → Compress → (Additional Effects) → Sync Audio & Video → Titles/Transitions → Colour‑Correct → Export.