audiocodec
I've read they were hardware audio codecs. But how are they different from mp3, aac codecs?
mp3 and aac codecs can also be implemented in hardware (can't they?).
What key differences do AC'97 and High Definition Audio have?
How are AC'97 and High Definition Audio used most of the time?
P.S.: This article mentions AC'97 for modems. Why does modem need Audio codec?
Some main definitions:
As you can see, we'll have to explain a few things here.
A codec is short for encoder/decoder, which basically just means the following: Data generated by an encoder can always be decoded by an appropriate decoder. This happens to be valid for video, audio, but you could also think about cryptography (an encoder needs an appropriate decoder to display an encrypted message).
Nowadays, when a video codec is specified, the institutions that take part in it usually only specify the syntax of the standard. For example, they will say: "The bitstream format has to be like this", "The 0x810429AAB here will be translated into that", etc. Often they supply a reference encoder and decoder, but how an encoder is then written to match such a format completely is up to manufacturers.
This is the reason why you will find so many encoders for the very same codec, and some of them even commercial.
Before we mix up terminology, let's take an example. Consider the case for H.264. The standard's name is H.264 – that's not the name of the actual encoder. Mainconcept is a very good commercial encoder, whereas x264 is a free and open source one. Both claim to deliver good quality, of course.
The mere fact that you can optimize encoding makes for a competition here. Both encoders will deliver a standardized bitstream that can always be decoded by a H.264-compliant decoder.
So, all in all, let's just say that an encoder will:
The bitstream is then multiplexed into a container.
The decoder will:
They both conform to a codec standard. That's all!
These days, you will probably only find videos encoded with the codecs I will mention below. Interestingly, almost all of them were created by the Motion Picture Experts Group (MPEG). But there are some other, royalty-free codecs as well, e.g. those created by Google or the Alliance for Open Media, which are competitors to the MPEG standards.
Note that "MPEG" can refer to both codecs and containers, as you will see below. This adds to the confusion, but just know that "MPEG" alone doesn't mean anything, e.g. "I have a file in MPEG format" is very ambiguous".
MPEG-2 is quite old. Its first public release is from 1996. MPEG-2 video is mostly used for DVDs and TV broadcasting, e.g. DVB-T or satellite, and legacy applications where compatibility is important. MPEG-2 videos are mostly found in an .MPG container.
This is probably the one that was used mostly to encode videos for the web in the mid-2000s, but it has been superseded in the meantime. It offers good quality at practical file sizes, which meant that you could burn a whole movie of 90 minutes length onto a 600 MB CD (whereas with MPEG-2 you would have needed a DVD, see my answer here). It doesn't work so well for HD or 4K content anymore.
Some encoders that output MPEG-4 Part 2 video are DivX, its open sourced ripoff XviD, and Nero Digital.
MPEG-4 Part 2 videos mostly come in an AVI container, but MP4 is also seen often.
This is also known as MPEG-4 Advanced Video Coding (AVC) or H.264; it is the most used codec today. It offers good quality at small file sizes and therefore is perfectly suited for all kinds of video for the Internet or mobile devices. You will find H.264 in almost every modern applications, from phones to camcorders. On Blu-ray disks, video is now encoded in H.264.
Some encoders for it are: x264, NVENC (from NVIDIA), Mainconcept. The videos mostly come in MP4, MKV or MOV containers.
Also called MPEG-H Part 2, this is the successor of MPEG-4 Part 10 / AVC / H.264. It is aimed at higher resolutions (up to 8K) and can offer up to 50% higher encoding performance (in terms of quality vs. bitrate) compared to H.264 (see this paper, for example).
The standard was published in 2013, and slowly, the codec is beginning to be used more and more, e.g., for IPTV or online video transmissions. HEVC is also used by Apple to store videos and images (using HEIF) on iOS. However, the fact that there are multiple patent pools associated with HEVC has many companies (almost all but Apple) shifting to royalty-free alternatives. HEVC is also not natively supported by all browsers, making it unusable for web streaming.
The best-known encoder is x265. There's also NVENC. The videos usually come in MP4 containers.
VP9 (the successor of VP8) is a codec mainly developed by Google. It is open and royalty-free, and implemented in many browsers. Its quality is almost as good as HEVC, and sometimes even better (see this paper by Netflix). VP9 is what you get when you watch YouTube on a browser that supports it.
VP9 can be encoded with the libvpx encoder, and it often comes in WebM or MKV containers.
Some companies got together to form an even stronger competitor to HEVC – but as a royalty-free alternative. AV1 will be the successor of VP9, and it is based on what was supposed to become VP10. It is backed by the Alliance for Open Media (founded by Amazon, Cisco, Google, Intel, Microsoft, Mozilla, and Netflix). Read more about it here.
The libaom encoder can be used to generate AV1 bitstreams, but it is still experimental.
Until now we've only explained the raw "bitstream", which is basically just really raw video data. You could actually go ahead and watch the video using such a raw bitstream. But in most cases that's just not enough or not practical.
Therefore, you need to wrap the video in a container. There are several reasons why:
For all of those reasons, container formats were invented, some simple, some more advanced. What they all do is "wrap" the video bitstream into another bitstream.
A container will synchronize video and audio frames according to their Presentation Time Stamp (PTS), which makes sure they are displayed at exactly the same time. It would also take care of adding information for streaming servers, if necessary, so that a streaming server knows when to send which part of the file.
Let's take a look at some popular containers.
You will find videos mostly wrapped in the following containers. There are other less popular ones as well, but as I said, mostly, it's those:
Audio Video Interleave — this is the most basic container, it's just there to interleave audio and video. It was written in 1992 and is still used today, but considered legacy, so do not use it anymore.
is also known as MPEG-4 Part 14 and is based on the QuickTime file format. This is the go-to format for H.264 video, but it also wraps HEVC, MPEG-4 Part 2 and MPEG-2.
This container might also wrap audio only, which is why you'll find so many .mp4 files which are no videos but rather AAC-encoded audio, also in .m4a files (just a different extension). The extension .m4v is usually taken for video bitstreams.
Matroska Video (MKV) is an open sourced and free file format that is often found nowadays, as it supports basically any codec, from H.264 to VP9, and of course also many audio codecs.
WebM is based on MKV and is primarily used for VP9 video and Opus audio – it is the container of choice for web streaming video when these codecs are used.
The Ogg container is the container of choice for the Theora video codec (and the Vorbis audio codec), also created by the Xiph.Org Foundation. It's also free and open source (just like the codec).
The Flash video format was created by Adobe, for use in their streaming applications. It isn't used that much anymore, as the way streaming is done has changed significantly over the last years.
Also, which of the following are codecs, which ones are file formats, and which ones are neither?
libavcodec and libavformat libraries for creating codecs and containers, respectively. Most of video tools you find today are based on it.On a side note:
Am I even using the correct terminology?
I guess once would prefer to specifically use "codec" and "container" instead of "format" to avoid misunderstandings. A format can theoretically be anything, because both codecs and containers specify a format (i.e. how data should be represented).
That being said, the FFmpeg terminology would be to use "format" for the container. This is also because of the distinction between:
libavcodec, the library for encoding/decodinglibavformat, the library for the containersLet me preface this with the fact that I have just started investigating AptX. However, it appears that there are a number of BlueTooth 4.0 USB dongles on the market with AptX support and of course their hardware-specific drivers. Google is your friend. ;-) Here is a sample:
http://www.kinivo.com/Kinivo-BTD-400-Bluetooth-4-0-Adapter/dp/B007Q45EF4
I've read good things about their BTC450 Bluetooth hands-free unit; maybe their other products are likewise excellent.
Best Answer
From Wikipedia:
AC'97:
HD Audio:
The reason the MC'97 is tied to the AC'97 is to provide interaction between the MC'97 modem and the AC'97 speakers/mic, for using it like a "voice modem".