بسم الله الرّحمن الرّحیم
ابزار فشرده ساز را کدک(Codec) نامند.
1) کدک صوتی بر دو نوع است:
1-1) کدک صوتی بدون افت کیفیت (Lossless)
WMA > Windows Media Audio
ALAC > Apple Lossless Audio Codec
FLAC > Free Lossless Audio Codec
کدک بدون افت کیفیت متناظر است با حجم بالای این فایل ها و طبعا گزینه مناسبی برای Live Video Streaming نیستند.
1-2) کدک صوتی با قبول افت کیفیت (Lossy)
سه جنبه ای که در این نوع کدک صورت می پذیرد عبارت است از:
الف - حذف صدای غیر قابل شنیدن برای انسان
ب - کاهش کیفیت اصواتی که به سختی قابل شنیدن هستند
ج - فشرده سازی سریع باقی داده ها
1. AAC: Advanced Audio Coding
Defined by MPEG-4, AAC is the most common audio codec. It improved upon the MP3 codec by offering the same audio quality at a lower bitrate. As long as you aren’t looking for an exact replication, AAC works great. This widely supported standard is used by YouTube, Android, iOS, and iTunes.
Two extensions of AAC exist: HE-ACC for low bitrates and AAC-LC for low delay. We suggest using HE-AAC whenever bandwidth is a concern, and AAC-LC for two-way communication.
2. MP3: MPEF-1 Audio Layer III
The MP3 codec revolutionized music consumption in the 1990s, prompting millennials near and far to swap their CD players for iPods. Nearly every audio-supported digital device in the world can play back the MP3 format, making it a viable option for live streaming. But because AAC offers superior compression, we’d recommend going with that.
3. Opus
Developed by the Xiph.Org foundation, Opus provides higher quality audio than any other lossy audio format. It’s open-source and royalty-free, but has yet to be widely adopted.
4. Vorbis
Also a non-proprietary alternative, Vorbis was designed to compete against closed codecs like AAC. Again, this codec lacks native support across devices. And because Opus outperforms Vorbis as far as quality goes, there’s no benefit to taking this route.
5. Speex
The Xiph.Org foundation designed this patent-free codec as an alternative to proprietary speech codecs. Like Vorbis, though, the Speex codec has been made obsolete by Opus.
6. G.711
G.711 was first released back in 1972. The codec works fine for real-time communication, but it isn’t exactly broadcast-quality.
G.711 is often used for VoIP calls made to and from regular phones.
7. AC-3 and E-AC-3: Dolby Digital and Dolby Digital Plus
The AC-3 and E-AC-3 formats are less advanced than AAC. The only benefit of these audio codecs is backwards compatibility on Dolby Digital equipment.
