Artikler / Filosoferende Hi-Fi / Bit Rates - hvad er det?

Bit Rates - hvad er det?

 
                          
 
 
 
 
Artikel af Arne Rodahl
  
 
 
Som navnet antyder, beskrives den hastighed, hvormed bit overføres fra et sted til et andet. Med andre ord måles der, hvor mange data der transmitteres i et givet tidsrum. Bit Rate måles normalt i bit per sekund ( bps ), kilobit per sekund ( Kbps ).  
Bit Rate kan medvirke til kvaliteten af en lydfil. 
 
Sample Rate
Den hastighed, hvormed vi tager prøver og afspille dem kaldes ”sample rate”. Til cd'er er der taget 44.100 hvert sekund. Idet vi er nødt til at gemme værdien af hver prøve, følger det, at højere sample rates ville kræve en masse mere lagerplads, hvilket kan blive et problem. Omvendt vil en højere sample rate direkte svarer til en mere nøjagtig gengivelse af det originale signal, og derfor bedre lydkvalitet. Så i sidste ende måtte kompromiser tages og en 44.100 sample rate blev anset til at være et fornuftigt kompromis for cd'er.  
 
Bit Rate
Når man har et overblik over sample rates, er man i en god position til at betragte ”bitrates”, da Sample Rate og Bit Rate er uløseligt forbundet. Hver prøve man betragter er repræsenteret af et digitalt nummer og hver digitalt nummer er digitale bit.  En ”bit” er et enkelt ciffer, enten (1) eller nul (0). Cd`ens digitale numre består af 16 bits, som giver op til 65.563, der skalleres, - d.v.s. over 65.000 forskellige lydstyrker! Da vi har to ører, er vi nødt til at gemme to sæt prøver, én til hvert øre (stereo) . For at beregne bithastighed for cd-lyd, ganger vi vores sample rate (44.100) med 2 (for stereo) og med 16 (antallet af bits i vores digitale tal): 44.100 x 2 x 16 = 1.411.200 bits per sekund (bps) = 1.411 kbps Vi har nu beregnet bithastigheden for standard-cd, som viser sig at være 1.411 kbps .  
 
Kompression
Hvis man for eksempel brugte en 1 GB CD-afspiller til at gemme musik med ovennævnte bit rate, ville man kun være i stand til at lagre 100 minutters musik, -  ikke meget!  Dette forårsagede udviklingen af MP3-systemet til reduktion af behovet for mængden af plads til lagring af musik. Der findes mange kompressions systemer, med MP3 som det mest populære system. Lignende systemer alle med det formål at reducere filstørrelsen . Hvordan komprimering fungerer er noget uden for rammerne af denne artikel, idet en mængde kompliceret matematik anvendes til beregning af hvilke dele af musik, vi kan høre, og hvilke dele vi ikke kan høre. Grundlæggende er det pladsbesparelse ved ikke at gemme bits af musik, som vi alligevel ikke kan høre. 
I MP3-kompressionprocessen fjernes typisk 90% af data og efterlader filstørrelser omkring en tiendedel af den oprindelige størrelse, hvilket betyder, at MP3-medier er i stand til at lagre ti gange antal sange i forhold til at gemme de originale ukomprimerede cd-data.
 
Bit Rate relateret til lydkvaliteten
Ved reduktion af filstørrelsen til omkring en tiendedel af den oprindelige størrelse, resterer kun en tiendedel af antallet af prøver (samples), hvilket reducerer bithastigheden tilsvarende. En typisk bithastighed for en jævn lydkvalitet ville være omkring 128 kbps (kilo bits per sekund, eller tusinde bits per sekund), svarende til ca. en tiendedel af bithastigheden for en standard cd. Desværre har kompression-systemet begrænsninger, - jo flere data der smides væk, des mindre fil-størrelse, des lavere bitrate, men dårligere lydkvalitet.
    
Skønnet vurdering af lydkvaliteten ved forskellige Bit Rates (kbps):  
 
HI-FI
9.216 kbps = 24 bit pr. sample / 192.000 samples
4.608 kbps = HD-CD (24 bit pr. sample / 96.000 samples)
1.411 kbps = CD og SONOS (16 bit pr. samples / 44.100 samples)  
 
Næsten HI-FI (MP3 komprimeret)
320 kbps = Tæt på den originale cd-lyd (net-radio : bl.a. WiMP hi-fi og Spotifi)
 
Ikke HI-FI (MP3 komprimeret)
128 kbps = Middelmådig lydkvalitet, typisk for MP3 (net-radio : bl.a. DR og mange andre)
64 kbps = Anbefales ikke til musik, men god til tale
32kbps = Dårlig, der ofte bruges til at reducere web download-tider 
 
 
                                _______________________________ 
 
 
 
Why a CD?
 
Some will question the seriousness of CD as a demo medium at all. Isn’t CD so last millennium in comparison with newer high resolution formats? Technically, the question may be relevant and deserves a considered response. CD quality, (stereo, 44.1 kHz sampling rate and 16-bit constant bit depth) has been criticised since the early 1980’s for limited bandwidth, restricted bit resolution and dynamic range of “only” 96 dB. Many early CD releases were irrefutably bad; cold, aggressive, anaemic bass, harsh top end, perhaps because the best recording and mastering engineers still focused on the LP. CD required a new skill set, continued development of digital recording technologies and new techniques for transferring and mixing analogue tapes.
 
With a steep learning curve, CD quality is now much improved.
 
96 dB dynamic range, properly captured, 20 Hz – 20 kHz frequency bandwidth, properly rolled off, etc., are more than up to the task of handling demanding performances with surprising verisimilitude. One could argue that the main value of high resolution formats is that they allow engineers and manufacturers of digital recording equipment more leeway in exploring the dynamics and extended frequency response of hi-rez media.
Extracting outstanding sound from CD is altogether more demanding. It should also be noted that many recordings come nowhere near testing the technical limits of CD.
 
Extreme dynamic compression is applied in pop genres for CD and download to boost levels to a constant high “loudness” to make music more “exciting” on cheap headphones, portable players or in a car. The sound of many modern performances and even remasterings of older recordings is compressed to a narrow 10 – 15 dB dynamic range, by turning down dynamic peaks and boosting quiet passages so that the track’s high uniform loudness grabs and holds the listener’s attention. This is quite a departure from the hi-fi goal of natural, engaging musicality. In this context, 96 dB of dynamic range is more than adequate and, properly deployed, can offer near perfection.
 
The day may well come when audiophiles look back in nostalgic gratitude that we were ever allowed to experience a widespread high quality medium like CD, before the mainstream consumer was steered towards MP3 and other heavily compressed formats vastly inferior to CD. Let us enjoy the wealth of musical treasures available to us now – Long live the CD!
 
Lars Worre, managing director, Dali
 
 
                               __________________________________
 
 
Giver det mening?
 
 
 
What about 16 bit vs. 24 bit audio?
It's true that 16 bit linear PCM audio does not quite cover the entire theoretical dynamic range of the human ear in ideal conditions. Also, there are (and always will be) reasons to use more than 16 bits in recording and production. None of that is relevant to playback; here 24 bit audio is as useless as 192kHz sampling. The good news is that at least 24 bit depth doesn't harm fidelity. It just doesn't help, and also wastes space.
 
Listening tests
Understanding is where theory and reality meet. A matter is settled only when the two agree. Empirical evidence from listening tests backs up the assertion that 44.1kHz/16 bit provides highest-possible fidelity playback. There are numerous controlled tests confirming this, but I'll plug a recent paper, Audibility of a CD-Standard A/D/A Loop Inserted into High-Resolution Audio Playback, done by local folks here at the Boston Audio Society. Unfortunately, downloading the full paper requires an AES membership. However it's been discussed widely in articles and on forums, with the authors joining in. Here's a few links:
 
 
 
 
This paper presented listners with a choice between high-rate DVD-A/SACD content, chosen by high-definition audio advocates to show off high-def´s superiority, and that same content resampled on the spot to 16-bit / 44.1KHz Compact Disc rate. The listners were challenged to identify any difference whatsoever between the two using an ABX methodology.
 
 
Boston Audio Society conducted the test using high-end professional equipment in noise-isolated studio listening environments with both amateur and trained professional listeners. In 554 trials, listeners chose correctly 49.8% of the time. In other words, they were guessing.
 
Not one listener throughout the entire test was able to identify which was 16/44.1 and which was high rate, and the 16-bit signal wasn't even dithered!
 
Another recent study investigated the possibility that ultrasonics were audible, as earlier studies had suggested. The test was constructed to maximize the possibility of detection by placing the intermodulation products where they'd be most audible. It found that the ultrasonic tones were not audible... but the intermodulation distortion products introduced by the loudspeakers could be. This paper inspired a great deal of further research, much of it with mixed results. Some of the ambiguity is explained by finding that ultrasonics can induce more intermodulation distortion than expected in power amplifiers as well. For example, David Griesinger reproduced this experiment and found that his loudspeaker setup did not introduce audible intermodulation distortion from ultrasonics, but his stereo amplifier did.
 
 
32 ...64 Bit ?
Der findes ingen 32-bit medier, endsige 64-bit. Men outputtet af et digital-filter i en DAC eller afspiller kan have 32-bit og tilføre disse data direkte til en DAC, der kan acceptere dem. Dette kan resultere i mindre forarbejdningstrin.
Betyder det noget? Sandsynligvis ikke.
 
John Atkinson Redaktør, Stereophile