定点的wma解压库-libwma

发布于 2009-08-20 07:50:11 浏览：12558 订阅该版

在2007 GSoC项目中，RockBox完成了libwma的定点解压库：
```
General
In the summer of 2006, Paul Jones began work on converting the ffmpeg SoundCodecWMA decoder to fixed point so that it could be used with Rockbox (FS#4984). Although he did not complete the project, it raised interest in eventually getting ffmpeg's decoder working in Rockbox. This Google Summer of Code 2007 project completed his task.

Status
Codec is currently in SVN and functioning well above real time on all software targets. Results appear comparable to ffmpeg for nearly all files. Noise coding and line spectral pairs are implemented, and appear to function as well as ffmpeg. Seeking has been implemented and appears robust. Performance is now extremely good on ARM. Coldfire performance is also very good but could be further improved.

Both WMA Std. flavors are supported. No special WMA variants are supported (Pro, Voice, Lossless) due to lack of an open source decoder.

Todo:
    * Additional ASM optimization (requantization, etc)
    * Review Tremolo and see if more ASM code can be used in WMA too
    * Improve the makefile to allow compiling a stand alone codec for testing

Who's working on this
    * Michael Giacomelli (saratoga)

with lots of help from:

* preglow
    * linuxstb (developed the ASF parser)
    * marsdaddy (Paul Jones) 
```
为了把它移植到这个网络收音机项目中来还需要修改其中的汇编代码，从ARM指令转换成Thumb2指令。

13 个回答

bernard 2009-08-21

这家伙很懒，什么也没写！

bernard 2009-09-11

这家伙很懒，什么也没写！

bernard 2009-09-11

这家伙很懒，什么也没写！

几个用到的变量（占用空间比较大的变量）
wmadeci.c中的
```c
fixed32 coefsarray[MAX_CHANNELS][BLOCK_MAX_SIZE] ; /* 16384 */
fixed32 frame_out_buf[MAX_CHANNELS][BLOCK_MAX_SIZE * 2] ; /* 32768 */

fixed32 stat0[2048], stat1[1024], stat2[512], stat3[256], stat4[128]; 
/* 大小分别为8192, 4096, 2048, 1024, 512，用于放fsincos的窗口，基本上是定值 */

uint16_t runtab0[1336], runtab1[1336], levtab0[1336], levtab1[1336];
/* 大小分别为2672, 2672, 2672, 2672 */

VLC_TYPE vlcbuf1[VLCBUF1SIZE][2]; /* 18392 */
VLC_TYPE vlcbuf2[VLCBUF2SIZE][2]; /* 14296 */
VLC_TYPE vlcbuf3[VLCBUF3SIZE][2]; /* 1440 */
VLC_TYPE vlcbuf4[VLCBUF4SIZE][2]; /* 2160 */
/* 用于放置vlc的一个快速查找表 */
```

这几个总计112000字节

而一个wma解码的上下文需要53400字节大小

wma解码时主要的函数调用流程：
`wma->asf_read_packet` (读取wma原始数据), wma_decode_superframe_init(获得帧头数据)->针对每个帧做解码:

`wma_decode_superframe_frame -> wma_decode_frame -> wma_decode_block -> mdct_backward (faster IMDCT), wma_window (Apply MDCT window and add into output)`

解码部分主要在`mdct_backward`，因为wma是基于MDCT的，需要做一个反变换。libwma是基于帧的方式进行解码，所以每次的数据缓存需要一个最大的超帧大小(superframe)