VOD Deep Dive Part 3: Audio Fundamentals — Making Sound Small

This is Part 3 of the VOD Streaming Deep Dive series.

How Sound Becomes Digital#

Sound is air vibration — a continuous waveform. Computers can only store numbers, not continuous waves. Two steps are needed:

1. Sampling: Measure the wave’s height at regular intervals
2. Quantization: Convert each measurement into a number

Amplitude
 ▲
 │  ●   ●                     ●    Sample points
 │     ●  ●  ●
 │           ●  ●
 │              ●  ●   ●
 └──────────────────────► Time
     ↑ ↑ ↑ Sample N times per second — N is the "sample rate"

Sample Rate#

Unit: Hz (hertz — samples per second)

Nyquist theorem: To reproduce a frequency F, you need a sample rate of at least 2F. Human hearing tops out around 20 kHz, so 44.1/48 kHz is just enough (with a small margin).

For VOD, standardize on 48 kHz. If your source is 44.1 kHz, resample during transcoding with -ar 48000.

Bit Depth#

How many bits per sample:

Most video audio is 16-bit, 48 kHz.

Channels#

A channel is an independent audio track:

5.1 surround layout (top-down view):

         FL ──── C ──── FR
              │  🧑  │
              │      │
         SL ──┻━━━━──SR
                LFE

Audio Bitrate: How Many kbps Is Enough?#

Audio bitrate is also bits per second, but much smaller than video — typically 5–10% of the video bitrate.

For VOD: stereo AAC at 128 kbps is the correct answer for the vast majority of scenarios.

Major Audio Codecs#

AAC (Advanced Audio Coding) — The Streaming Default#

• By: MPEG (same organization behind H.264)
• Year: 1997
• Compatibility: every video platform, browser, and phone
• Variants:
  • AAC-LC (Low Complexity): Most common. HLS/DASH default.
  • HE-AAC (High Efficiency): Better at low bitrates (<64 kbps)
  • HE-AAC v2: HE-AAC + parametric stereo, decent at 48 kbps

MP3 — Retired#

Classic but less efficient than AAC. Original patents expired in 2017. No reason to use MP3 in new projects.

Opus — The Web Newcomer#

• Open-source, royalty-free
• Excellent from 6 kbps (voice) to 510 kbps (music)
• WebRTC default, used by Discord
• But HLS/DASH compatibility lags behind AAC; limited iOS/Safari support

Dolby Family — Cinema-Grade#

Dolby Atmos on Netflix, Disney+, and Apple TV+ is a hallmark of premium subscriptions.

FLAC / ALAC — Lossless#

Lossless compression reduces size by 50–70% while perfectly preserving the original PCM data. Used in Apple Music lossless tier and audiophile contexts. Not practical for video streaming — bitrate is too high.

Multi-Language Audio Tracks#

A single video file can carry multiple audio tracks:

MP4 file
├── video track   (H.264)
├── audio track 1 (AAC, English)
├── audio track 2 (AAC, Chinese)
├── audio track 3 (AAC, Japanese)
└── subtitle track (WebVTT)

Streaming protocols (HLS/DASH) support independent audio track delivery — the player only downloads the language the user selected:

#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="audio",LANGUAGE="en",NAME="English",DEFAULT=YES,URI="audio/en/index.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="audio",LANGUAGE="zh",NAME="中文",URI="audio/zh/index.m3u8"

More on this in Part 5: Streaming Protocols.

Loudness Normalization#

Ever noticed the volume spike when a commercial cuts in? That’s because different content has wildly different loudness levels.

Loudness normalization adjusts all content to a uniform perceived loudness level (not peak volume).

Common Standards#

LUFS (Loudness Units Full Scale) is the international standard for perceived loudness.

ffmpeg Loudness Normalization#

# Normalize audio to -14 LUFS
ffmpeg -i input.mp4 -af loudnorm=I=-14:TP=-1.5:LRA=11 -c:v copy output.mp4

Hands-On: Inspect and Transcode Audio#

Check audio tracks in a video#

ffprobe -v error -show_streams -select_streams a input.mp4

Typical output:

codec_name=aac
sample_rate=48000
channels=2
channel_layout=stereo
bit_rate=128000

Standardize to AAC 48 kHz 128 kbps stereo#

ffmpeg -i input.mov \
  -c:a aac -b:a 128k -ar 48000 -ac 2 \
  -c:v copy \
  output.mp4

• -c:a aac: Audio codec AAC
• -b:a 128k: 128 kbps bitrate
• -ar 48000: 48 kHz sample rate
• -ac 2: 2 channels (stereo)
• -c:v copy: Copy video stream as-is (saves time)

Key Takeaways#

1. Digital audio requires sampling rate (temporal density) and bit depth (amplitude precision).
2. VOD default: 48 kHz sample rate, 16-bit depth.
3. Consumer streaming defaults to stereo (2.0); cinema uses 5.1 / Atmos.
4. AAC-LC at 128 kbps is the default audio setting for VOD.
5. A single video file can carry multiple audio tracks (multi-language).
6. Loudness normalization (EBU R128 / -14 LUFS) prevents the “ads are too loud” problem.

Previous: Part 2: Video Codecs

Next: Part 4: Container Formats — MP4, fMP4, and CMAF