RealAudio voice encoder options don't work

[jwinter]

I am trying to choose a good format to distribute recorded lectures over the net and wanted to try various compression rates to compare the quality etc. However with dMC set to voice, and at the low rates I am interested in, all choices produce the same file size. I downloaded the free Helix encoder and it gave me at least 3 choices to try with their voice encoder - 8.5kbps, 16kbps, and 32kbps. Unfortunately the super slow rates of 5kbps and 6.5kbps are not enabled unless you buy the professional package. So why doesn't dMC support bit rates other than 16kbps under voice option. (If I select 56k modem target audience - which is surely fast enough for 32kbps - why does it still use 16kbps). And how about allowing one to select the bit rate rather than the terribly vague choice of "target audience"?

Also when pointing at a RealMedia file in Windows Explorer why is the information displayed about the file so lacking - basically no information at all - just zeroes are displayed ?

[ChristinaS]

Re: RealAudio voice encoder options don't work

I am trying to choose a good format to distribute recorded lectures over the net and wanted to try various compression rates to compare the quality etc. However with dMC set to voice, and at the low rates I am interested in, all choices produce the same file size. I downloaded the free Helix encoder and it gave me at least 3 choices to try with their voice encoder - 8.5kbps, 16kbps, and 32kbps. Unfortunately the super slow rates of 5kbps and 6.5kbps are not enabled unless you buy the professional package. So why doesn't dMC support bit rates other than 16kbps under voice option. (If I select 56k modem target audience - which is surely fast enough for 32kbps - why does it still use 16kbps). And how about allowing one to select the bit rate rather than the terribly vague choice of "target audience"?

Also when pointing at a RealMedia file in Windows Explorer why is the information displayed about the file so lacking - basically no information at all - just zeroes are displayed ?

The target audience is not the bit rate but rather the internet connection speed. It refers to a 28k modem, or 56k modem, etc. The actual bit rates will be quite different, but always what's appropriate for those connection speeds.

The reason target audience is used instead of bit rate is that Real Player is made for streaming media. That is the terminology they use, so there's not much point in referring to things in any other way.

If you encode to Real Media at 8000Hz, mono, target audience 28k surestream you get 16kbps as a maximum bit rate, but it will include several substreams:
16 Kbps - RealAudio (RealAudio 5.0) 16000 Khz
8.5 Kbps - RealAudio (RealAudio 4.0) 8000 Khz
6.5 Kbps - RealAudio (RealAudio 4.0) 8000 Khz
16 Kbps - RealAudio (RealAudio 5.0) 16000 Khz - Backwards Compatibility Stream

If you encode to single rate you also get 16kbps as a maximum bit rate, but a file about 3 times smaller than surestream. Surestream allows for encoding for several target audiences in the same file. The resulting file, while larger than one produced for one target audience or the other, can be streamed optimally on either types of connection, as long as it's done through a streaming server. For a progressive stream (i.e. regular servers using the http protocol), there's no size advantage, on the contrary. So single rate is best for that case.

As for why the lower bit rates aren't available individually, I don't know, I assume that's because the codecs that could produce them are not publicly available to dMC.

If you convert to wma 5 kbps, mono, 8KHz, CBR, you'll use 1/3 of the space of the smallest .rm file - or a compression of 236:1 .

Real Media files do not contain ID tags, which is why you don't see that information. However, if you play an .rm file through Real Player, you can see the clip source (File > Clip properties > Clip SOurce) and you get all the information that Real Player has about it, which is quite comprehensive.

[jwinter]

Re: RealAudio voice encoder options don't work

Thanks for the good information Christina.

I was aware that the target audience referred to the connection speed, but I would argue that the actual bit rate used by dMC is not appropriate in the case of selecting a 56k modem. This rate is easily fast enough to support 32kbps (which is what is used if this selection is made in the free "RealProducer" encoder - as well as a 26kbps substream for speed fallback).

Incidentally I thought I read somewhere that Windows had purchased the rights to RealAudio's codecs and that WMA version 9 was to some extent a repackaging of Real's product? However the low bit rate audio encoding selections available under the two systems don't seem to match.

It seems odd that the information displayed about the bit rates etc should only be read from the ID tags. Surely dMC gets other information from within the file content, why not the bit rates, compression, etc?

Maybe only reading the tags is why dMC gets so much popup information wrong? I noticed that MP3 file size and compression info is totally wrong, often displaying 1:1 compression, but sometimes random other numbers apparently calculated by assuming some random original file size. Even the "encoded by" information is often wrong!

With Windows Media Audio files, the compression factor would be approximately correct except that it assumes a stereo file to start with - so it is a factor of 2 out.

However with MP4 files one doesn't get a popup at all. I assume this has to be a bug because surely MP4s have tag info ?

Not that I wish to complain - dMC is a really useful product and at the best possible price!

[ChristinaS]

Re: RealAudio voice encoder options don't work

The compression ratios are based on the standard wav file size at cd quality settings: 16-bit, 44.1KHz, 2-channel stereo. That would be for uniformity of comparisons.