Audiovisual digital documents Adolf Knoll National Library of the Czech Republic adolf. knoll@nkp. cz
Digitally born vs. digitized n n New digital audio recordings New digital video recordings n n n Digitized analogue audio recordings Digitized movies Reformatted digital documents for better computer use
Compressed vs. uncompressed n n Only sound recordings can be stored in uncompressed digital form / WAVE or CDA formats Video is always compressed
Compressed audio and video n n MPEG initiatives / it provides compression solutions for both MPEG ¡ ¡ Intra-frame coding technique Extra-frame coding technique
Intra-Frame Coding
Non-Intra Frame Coding
How it works… I – Intra-Frame P – Predicted Future Frame B - Bi-directional interpolated prediction frames
MPEG-1 for audio n n Started in 1988 3 operating modes/layers Without data reduction more than 1400 kbps for 1 second of stereo signal = 175 KB, i. e. 1 hr = 630 MB Compression ratios 1: 4 (layer one) ¡ 1: 6… 1: 8 (layer two) ¡ 1: 10… 1: 12 (layer three) – 128… 112 kbps … to preserve CD quality MP 3 comes from here ¡
MP 3 – typical performances n n n Telephone sound…. 8 kbps (96: 1) Better short-wave radio…. 16 kbps (48: 1) FM radio…. 56… 64 kbps (16: 1) Near-CD…. 96 kbps (16: 1 – usually free in most codecs) CD…. 112… 128 kbps
MPEG-1 video n n n Used until today as the format for the so-called Video. CD It is comparable to the perception of a VHS analogue recording Almost all DVD players can play also Video. CD It does not use complex encoding methods MPEG-1 was finalized in 1991, and was originally optimized to work at video resolutions of 352 x 240 pixels at 30 frames/sec (NTSC based) or 352 x 288 pixels at 25 frames/sec (PAL based), commonly referred to as Source Input Format (SIF) video.
MPEG-2 n n n MPEG-2 was finalized in 1994 for digital television broadcasting, such as the efficient coding of fieldinterlaced video. 720 x 480 resolution video at 30 frames/sec, at bitrates up to 15 Mb/sec for NTSC video HDTV resolution of 1920 x 1080 pixels at 30 frame/sec, at a bit-rate of up to 80 Mb/sec Used for DVD movies Used also for Super. Video. CD (SVCD) media that can be played in some DVD players (not all of them) – but here with lower resolution
MPEG-2 Advanced Audio Coding n n n AAC – another perceptual coding scheme Better prediction algorithm 5 channels as in cinema today C L R L – left R – right C – centre LS – left-surround RS – right-surround LS RS Used for DVD
Encoding CD audio n n Both MP 3 and AAC can maintain near. CD quality – AAC 50% more data for the same media, i. e. it can do at 96 kbps what MP 3 does at 128 kbps Other formats: WMA and MP 3 pro ¡ Similar acoustic impression at 64 kbps as MP 3 at 128 kbps
MP 3 pro n 3 parts of signal: ¡ ¡ ¡ 0 -8. 1 k. Hz (usual MP 3 coder) – it is seen by ordinary players 8. 2 -16. 3 k. Hz (calculating average power per frame) more than 16. 3 k. Hz (deleted) When decoding the middle part is selected the middle frequency part of it (4. 1 -8. 1 k. Hz) and raised up by a pitch up to 8. 2 -16. 3 k. Hz Good for average players and music with less high tones (no high bitrate support, synthese of high frequences from low ones)
MPEG-4 Added e. g bitrate scalability and objectbased representation
Media n n n DVD - commercial: MPEG 2 with AAC; 3 – 8 mbps; 720 x 480 in PAL; 30 -70 MB/min (15 -20 min per CD) VCD – MPEG-1; 1. 15 mbps; 352 x 288 in PAL; 10 MB/min (74 min per CD) SVCD – MPEG-2; variable up to 2. 6 mbps; 480 x 576 in PAL (MPEG-1, layer II); 10 -20 MB/min (35 -60 min per CD)
Other possibilities Div. X or xvid codecs ¡ ¡ ¡ MPEG-4 in very high quality and usually MP 3 for sound (but also other sound format) – stored in AVI container High resolution and high quality DVD back-up – usually 2 CDs per one movie Can be used for making VCD or SVCD
How to backup a DVD n n n Rip the files onto hard disk Calculate the desirable bitrate keeping in view the size of media for storage Encode Cut if necessary into parts for storage on more media units Free software tools available for this
To compare… VCD vs. DIVx vs. SVCD All of them compressed to store a movie (1 hr 30 mins) on 2 CDs
Usage of audio and video on the web n n n For creating better research environment Taking care of legal approach Video sequences or audio files in higher compression can be referenced from web pages, downloaded/streamed, and enjoyed
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