Taping/Trading FAQ

[Phrazz]

OK...time to get this thread started.

Let's put together a comprehensive reference for taping and trading,
including everything from the recording process itself, recommendations
for gear and techniques, etiquette and club/band policies as well as a
list of all Slip source sites. Cleantone and Jeff have the most experience
with these items and can give us "tricks of the trade", then we can take
this raw information and present it in a nice page or two so people can
just refer to this link for any questions (we can add links to sites that
already do a good job with gear recommendations, like USENET rec.audio
FAQs and such).

So let me start off with a simple question:

What are the most popular audio formats and what are the pros and cons?

Ok, that's a simple question, but we gotta start somewhere....

-Phrazz

[Cleantone]

Well, I could type all the info out but there are so many good references already put together online... I'd also try to field any specfic questions and/or break some of this down to laymans (sp?) terms.

Lets talk digital first...

Digital Audio Formats:
http://www.teamcombooks.com/mp3handbook/12.htm

Digital audio comes in many different formats, and multiple formats will be a fact of life for the foreseeable future. Groups like MPEG have created open standards, but even formats based on the same MPEG standard may not compatible with each other because of proprietary components.

Fortunately for consumers, many hardware and software players are able to support multiple formats—so if you purchase digital music in any of the major formats (MP3, WMA, etc.) you will be in a good shape. If a format does become obsolete, plenty of tools are available for converting digital audio to different formats.

Digital Audio Files
An audio file has two main parts: a header and the audio data. The header is used to store information about the file, including the resolution, sampling rate and type of compression. Often a “wrapper” is used to add features, such as license management information or streaming capability, to a digital audio file.

The format of a digital audio file refers to the type of audio data within the file. The file type refers to the structure of the data within the file. It is common for the same format to be used by more than one file type. For example, the PCM format is found in both WAV and AIFF files.

Table 1 - Common Digital Audio Formats

Type / Extensions / Codec

AIFF (Mac) / .aif, .aiff / *PCM

AU (Sun/Next) / .au / *u-law

CD audio (CDDA) / N/A / PCM

MP3 / .mp3 / MPEG Audio Layer-III

Windows Media Audio / .wma / Proprietary (Microsoft)

QuickTime / .qt / Proprietary (Apple Computer)

RealAudio / .ra, ram / Proprietary (Real Networks)

WAV / .wav / *PCM

* Can be used with other codecs.

WAV
WAV is the default format for digital audio on Windows PCs. WAV files are usually coded in PCM format, which means they are uncompressed and take up a lot of space. WAV files can also be coded in other formats, including MP3.

AIFF and AU
AIFF is the default audio format for the Macintosh, and AU is the default format for SUN systems. Both of these formats are supported on most otherplatforms and by most audio applications. Each of these formats can be compressed, but compression sometimes creates compatibility problems with other platforms.

Streaming Audio
Streaming audio avoids many of the problems of large audio files. Instead of having to wait for the entire file to download, you can listen to the sound as the data arrives at your computer.
Streaming audio players store several seconds worth of data in a buffer before beginning playback. The buffer absorbs the bursts of data as they are delivered by the Internet and releases it at a constant rate for smooth playback.

Many digital audio formats can be streamed by wrapping them in a streaming format, such as Microsoft ‘s ASF (Active Streaming Format), which can be used to stream MS Audio, MP3 and other formats.

Standard Formats
Standard formats make it easier for software developers and equipment manufacturers to produce products that are less costly and more compatible with each other. The compatibility provided by standard formats helps assure consumers that their music and equipment won’t become obsolete. Cassette tapes, compact discs and PCM are examples of standard audio formats that benefit both consumers and manufacturers.

PCM
PCM (Pulse Code Modulation) is a common method of storing and transmitting uncompressed digital audio. Since it is a generic format, it can be read by most audio applications—similar to the way a plain text file can be read by any word-processing program. PCM is used by Audio CDs and digital audio tapes (DATs). PCM is also a very common format for AIFF and WAV files.

PCM is a straight representation of the binary digits (1s and 0s) of sample values. When PCM audio is transmitted, each “1” is represented by a positive voltage pulse and each “0” is represented by the absence of a pulse. Figure 26 shows how binary data is converted to a PCM signal.
DPCM

DPCM (Differential Pulse Code Modulation)  is a simple form of lossy compression that stores only the difference between consecutive samples. DCPM uses 4 bits to store the difference, regardless of the resolution of the original file. With DCPM, an 8-bit file would be compressed 2=1, and a 16-bit file would be compressed 4=1.

ADPCM
ADPCM (Adaptive Differential Pulse Code Modulation)  is similar to DCPM except that the number of bits used to store the difference between samples is varied depending on the complexity of the signal. ADPCM works by analyzing a succession of samples and predicting the value of the next sample. It then stores the difference between the calculated value and the actual value.

u-law Compression
u-law (pronounced “mew-law”) is a common lossy compression scheme, similar to ADPCM, which can be used on AU, AIFF and WAV files.

MPEG Audio
MPEG Audio is a family of open standards for compressed audio that includes MP2, MP3 and AAC. (See Chapter 13 for more detailed information on MPEG Audio.)
MPEG-Based Proprietary Formats
Several proprietary formats are based on MPEG audio. Some of these are used in special applications, such as voice mail systems, high definition TV and satellite radio. Others compete directly with MP3 and are based on AAC or MP3, with proprietary wrappers. The sound quality of some of these is very good, but their proprietary nature makes them incompatible with many programs and portable players.
a2b
AT&T’s a2b music is a sophisticated music distribution system with many features, such as watermarking and encryption, to support copyright protection and royalty tracking. It is based on the MPEG-2 AAC Low Complexity Profile. The Policy Maker feature of a2b is a flexible electronic licensing system, which can control how music is used and distributed. Music encoded with a2b can include artwork, credits, lyrics and links to the artist’s Web site.

Proprietary Formats
Even though MPEG Audio formats are based on open standards and widely used, many companies continue to develop proprietary digital audio formats. Proprietary formats can generate enormous profits for the developer if the format becomes a de facto standard. The marketplace, which tends to favor open standards like MPEG, will ultimately decide which formats will prevail.

MP4
Global Music Outlet uses the term MP4 to describe its proprietary music delivery system. It’s based on an enhanced version of MPEG AAC and includes an embedded player (each song is an .EXE file). Album graphics and links to the artist’s Web site can be embedded in the file.

Liquid Audio
Liquid Audio is a sophisticated music distribution system based on Dolby Digital and MPEG AAC. It supports both downloadable and streaming audio and uses watermarking and encryption for copyright protection. Music encoded with Liquid Audio can include artwork, lyrics, notes and pricing, along with links to a Web site where the song or album can be purchased. Liquid Audio has a playlist feature and allows you to burn songs to a CD if you have a supported CD-R drive.

Apple QuickTime
QuickTime is a widely used multimedia format from Apple Computer that supports both streaming audio and streaming video. Much of the MPEG-4 standard is based on QuickTime, and it is widely used for streaming video on the Web.

Non-MPEG Proprietary Formats
Several digital audio formats exist that are entirely proprietary. Many of these are quite good and are widely used.
Dolby Digital (Formerly AC-3)
Dolby Digital is a very high quality audio encoding and noise reduction system that is the audio component of High Definition Television (HDTV) and digital broadcast TV (DTV). It is also used in DVDs, laser discs, digital cable and direct broadcast satellite (DBS) systems.

EPAC
EPAC is a perceptual audio encoding scheme based on PAC—developed by Bell Labs, the research and development arm of Lucent Technologies. EPAC is reported to produce quality indistinguishable from the original CD at 128 kbps. However, I participated in one listening test where the audience was able to consistently tell the difference between original CD tracks and the same tracks encoded in EPAC at 160 kbps.

Windows Media Audio
Microsoft’s Windows Media Audio (WMA) format is a relatively late entry into the field of proprietary audio formats. WMA performs very good at lower bit-rates and is reported to produce quality indistinguishable from the original CD at 128 kbps. WMA is supported by most full-featured player programs and by many portable players. WMA is royalty-free when incorporated into software that runs on the Windows platform.

RealAudio
RealAudio was the first widely used system for streaming audio and video over the Internet. It is a proprietary format, but it is used by many online music stores for sample clips of songs. The RealPlayer also provides support for MP3.

TAC
TAC (Transparent Audio Compression) is a high-quality perceptual encoding scheme developed by K+K Research. TAC uses Adaptive Bit-rate Management (ABM), which is similar to VBR (variable bit-rate) encoding. TAC was developed as part of K+K Research’s MP02 (Music Publisher 02) software.

TwinVQ (VQF)
TwinVQ (Transform-domain Weighted Interleave Vector Quantization) is an encoding scheme developed by the NTT Human Interface Lab in Japan. TwinVQ is reported to provide higher quality than MP3, but encoding times are reported to be much longer, and CPU utilization is reported to be higher during playback.

[Cleantone]

Now that your all experts on digital audio

We'll look at data compression formats:
http://en.wikipedia.org/wiki/Audio_compression

Audio data compression
Audio compression is a form of data compression designed to reduce the size of audio data files. Audio compression algorithms are typically referred to as audio codecs. As with other specific forms of data compression, there exist many "lossless" and "lossy" algorithms to achieve the compression effect

Lossless compression
Compared with image compression, lossless compression algorithms are not nearly as widely used in audio compression. The primary users of lossless compression are audio engineers and those consumers who disdain the quality loss from lossy compression techniques such as Vorbis and MP3.

First, the vast majority of sound recordings are natural sounds, recorded from the real world, and such data doesn't compress well. In a similar manner, photos compress less efficiently with lossless methods than computer-generated images do. But worse, even computer generated sounds can contain very complicated waveforms that present a challenge to many compression algorithms. This is due to the nature of audio waveforms, which are generally difficult to simplify without a (necessarily lossy) conversion to frequency information, as performed by the human ear.

The second reason is that values of audio samples change very quickly, so generic data compression algorithms don't work well for audio, and strings of consecutive bytes don't generally appear very often. However, convolution with the filter [-1 1] (that is, taking the first difference) tends to slightly whiten (decorrelate, make flat) the spectrum, thereby allowing traditional lossless compression at the encoder to do its job; integration at the decoder restores the original signal. More advanced codecs such as Shorten, FLAC and TTA uses linear prediction to estimate the spectrum of the signal. At the encoder, the estimator's inverse is used to whiten the signal by removing spectral peaks while the estimator is used to reconstruct the original signal at the decoder.

Lossless audio codecs have no quality issues, so the usabilty can be estimated by

* Speed of compression and decompression
* Compression factor
* Software support

This link compares various compression formats:
http://members.home.nl/w.speek/comparison.htm

and a confusing chart here:
http://web.inter.nl.net/users/hvdh/lossless/All.htm

Lossy compression
Note: Actually this is not a compression (i.e. redundancy reduction = reversible), but an irrelevance coding (i.e. an irrelevance reduction).

As opposed to lossless compression, where information redundancy is reduced, most lossy compression reduces perceptual redundancy; sounds which are considered perceptually irrelevant are coded with decreased accuracy or not coded at all.

In order to determine what information in an audio signal is perceptual irrelevant, most lossy compression algorithms use transforms such as the modified discrete cosine transform (MDCT) to convert sampled waveforms into a transform domain. Once transformed, typically into the frequency domain, component frequencies can be allocated bits according to how audible they are. Audibility of spectral components is determined by first calculating a masking threshold, below which it is estimated that sounds will be beyond the limits of human perception.

The masking threshold is calculated using the absolute threshold of hearing and the principles of simultaneous masking - the phenomenon wherein a signal is masked by another signal separated by frequency - and, in some cases, temporal masking - where a signal is masked by another signal separated by time. Equal-loudness contours may also be used to weight the perceptual importance of different components. Models of the human ear-brain combination incorporating such effects are often called psychoacoustic models or "psycho-models" for short.

Other types of lossy compressors, such as the linear predictive coding (LPC) used with speech, are source-based coders. These coders use a model of the sound's generator (such as the human vocal tract with LPC) to whiten the audio signal prior to quantization. LP may also be thought of as a basic perceptual coding technique; reconstruction of an audio signal using a linear predictor shapes the coder's quantization noise into the spectrum of the target signal, partially masking it.

Due to the nature of lossy algorithms, audio quality suffers when a file is decompressed and recompressed (generational losses). This makes lossy-compressed files unsuitable for audio engineering applications, such as sound editing and multitrack recording. However, they are very popular with end users (particularly MP3), as a megabyte can store about a minute's worth of music at adequate quality.

[Phrazz]

Damned, dude...if I'm not an expert already, I will be soon when I read and digest all of this. If recording is so easy, why doesn't everyone do it?

I am kidding of course. There's mics, the outboard gear, type of deck, other tips and tricks, not to mention all the mastering and mixing s/w. But you can't tell us everything or we'll all just buy gear and do it ourselves! [Jes, dis stuff ain't cheap, or cheap makes bad recordings, like my crappy MD with an obsolete mic which I'm going to sell on eBay real cheap!]

Thanks for the info -- this is very useful and will at the very least help people appreciate all the different methods and issues involved and know that it's not a walk in the park by any means (maybe with a dog gnawing at your leg). BTW, do you have the Thu. Knit show online yet?

[Cleantone]

BTW, do you have the Thu. Knit show online yet?

You mean Brad solo this coming Thursday? Yeah there should be a torrent of it somewhere already.

[Phrazz]

I figured I'd pre-empt the pre-grovels that are sure to come.

BTW, if you want to stickify the FAQ topic, please do it up. That's another Mod function that's quite useful.

[DCDireWolf]

check out this website:

taperrsection.com

comprehensive forum with very good faq's on this very topic.