The Secret Lives of MP3 Files

The Secret Lives of MP3 Files Doug Kaye The Conversations Network and GigaVox Media

Formats & Encoders • Lossless (WAV, AIFF) • Lossy - MPEG 1, Layer 3 (MP3) - AAC (AAC, M4A, M4B) - MPEG I, Layer 2 (MP2)

MPEG Confusion • Lossy Perceptual/Psychoacoustical Codecs • MP3 = MPEG-I Layer 3 • MP2 = MPEG-I Layer 2 (not MPEG-II)

Motion Picture Experts Group • MPEG-1:Video CDs, MP3 Audio • MPEG-2: Digital TV, Set-Top Boxes • MPEG-4: Online Multimedia (Video) • MPEG-7: Audio and Video Search • MPEG-21: Multimedia Framework

MPEG-1 for Geeks • Layer 1 • Simple 32-Band Algorithm • Philips DCC (Digital Compact Cassette) • Layer 2 (a.k.a. MUSICAM) • Also 32 Bands • International Standard for Broadcasting

MPEG-1 Layer 3 (MP3) for Geeks • Psychoacoustic Masking • 32 Bands Divided into 576 Subbands • More Accurate Masking Thresholds • Redundancy Reduction • Lossless Huffman Encoding • Bit-Reservoir Buffering • Joint Stereo

Sample Rate for Geeks • The Nyquist Theorem • Sample at 2x the Highest Frequency • 22.05kHz Sample Rate for 11kHz Audio • Sample Rate Is aSource (WAV or AIFF) Property of Uncompressed

Sample Rate in Practice • Standardize on 44.1kHz Sample Rate • Flash & Other Players Require n*11.025kHz • Resample if Source is 48kHz from DVDs

Bit Rate for Geeks • Independent of Sample Rate • Specifies Encoder Output File Size (CBR) • @64kbps, 1 hour ≈ 27MB • Variable Bit Rate (VBR) • For Higher Bit Rates Only • Not Universally Supported (Avoid It)

Bit Rate in Practice • “Use Higher Bit Rates for Music?” • It’s a Myth! • Human Voices Are Complex • Music Masks Its Own Artifacts • 64kbps is Most Common Today • 96kbps is Gaining

Podcasting Bit-Rate History • June 2003: 32kbps. “Files too large” • April 2004: 48kbps. “No problem” • September 2004: 64kbps. “Quality is low” • Today: Still 64kbps. • Tomorrow??

Stereo Encoding • “Stereo MP3s are twice as large as mono.” • It’s a Myth! • Only Bit Rate Specifies Output File Size • You May Want to Use Higher Bit Rates for Stereo

Stereo Encoding for Geeks • Dual Channel or Independent Channel (IC) - Entirely Separate Left and Right • But Most L/R Information is Redundant • Intensity Stereo (IS) • Mid/Side Stereo (MS) • Joint Stereo (JS) Allows IS/MS Combination

Stereo Encoding (Even Geekier) • JS Encodes L+R and L-R • If L=R then L-R=0 • SinceUsesRate is ConstantStereo Information Bit L=R Fewer Bits for

Stereo Encoding in Practice • StereoReason to (not Music vs.Voice) is a vs. Mono Good Use Higher Bit Rates • Greater Separation Suggests Higher Rates • If Mostly Speech, Consider 100% Mono • If Mono, Make L&R Digitally Identical • Always Encode in Stereo for Compatibility

Mastering for MP3 • Help the Encoder: Eliminate Unnecessary Data - High-Pass Filter at 80Hz - Low-Pass Filter at 11kHz (@64kbps encoding) - Normalize

Which is Louder? • It’s Not the Height of the Peaks (voltage) • It’s the Area Under the Curve (power)

Loudness • What’s the Standard? • We Asked: - Podcasters - Audio Engineers - Radio Engineers • Answer: There Isn’t One • It’s a Hard Problem to Solve

Normalization • Peak Normalization (common) - Maximizes Voltage, not Power • RMS Normalization - Maximizes Power (=Loudness) • Determine a Standard Loudness Level

Avoid Recording to MP3! • MP3 is a final/release format. • Not designed to be decoded and re-encoded. • Use MP2 Instead... • or the highest MP3 bit rate possible.

AAC/M4B Files? • Yes, AAC is Better Than MP3 • We Added AAC to Support iPod Bookmarks • Painful: Only iTunes Could Encode M4B • Doubled Much of Our Workflow • Can’t Be Easily Assembled

MP2: Why and When? • MPEG-1 Layer 2 • Designed as an Intermediate Format • The Standard in Broadcast Radio • 128kbps per Track • 44.1kHz Sample Rate Preferred

Audio Lessons Learned • MP3 Options • Audio-File Myths • RMS Normalization (Loudness) • AAC/M4B Files (iTunes & iPods) • MP2 Files

To Summarize • Record at 44.1kHz Sample Rate (not in MP3!) • Mastering - RMS Normalization (Pick a Standard Level) - 80Hz Hi-Pass, 11kHz Low Pass (for voice) - If Mono, Make L&R Digitally Identical • Encoding - 64kbps when L=R - Consider ≥96kbps for L≠R - Always Use Joint Stereo

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The Secret Lives of MP3 Files

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