[FFmpeg-devel] [PATCH 1/2] spdifenc: IEC 61937 encapsulation of DTS-HD for HDMI

[Jean-Daniel Dupas]

Le 27 janv. 2011 ? 20:06, M?ns Rullg?rd a ?crit :

> Anssi Hannula <anssi.hannula at iki.fi> writes:
> 
>> On 27.01.2011 19:50, Justin Ruggles wrote:
>>> Hi,
>> 
>> Hi!
>> 
>>> On 01/21/2011 01:32 PM, Anssi Hannula wrote:
>>> 
>>>> ---
>>>> 
>>>> This time the default mode of operation is to passthrough DTS-core only
>>>> unless the HD mode is explicitely requested by the user via an AVOption.
>>>> 
>>>> This way we avoid exporting the final bitrate in avctx->bit_rate as I
>>>> previously planned, as now the caller always specifies the output rate
>>>> themselves and therefore already knows the output rate (as is the case
>>>> for other formats as well).
>>>> 
>>>> An AVOption is used instead of the previously planned muxrate as the
>>>> usage doesn't exactly match muxrate semantics.
>>>> 
>>>> Changelog              |    2 +-
>>>> libavformat/avformat.h |    2 +-
>>>> libavformat/spdifenc.c |  147 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>> 3 files changed, 149 insertions(+), 2 deletions(-)
>>>> 
>>>> +static int spdif_header_dts4(AVFormatContext *s, AVPacket *pkt, int core_size,
>>>> +                             int sample_rate, int blocks)
>>>> +{
>>>> +    IEC61937Context *ctx = s->priv_data;
>>>> +    const char dtshd_start_code[10] = { 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xfe, 0xfe };
>>> 
>>> 
>>> dtshd_start_code could be made static.
>> 
>> Hm, what difference would it make?
>> 
>> Since it is a constant, the values are retained over calls by definition
>> even without "static" keyword, and indeed there seems to be no
>> difference between "const" or "static const" inside a function in output
>> of gcc.
> 
> It does make a difference:
> 
> ---8<---
> int baz(const char *);
> 
> int foo(void)
> {
>    const char x[] = { 1, 2, 3, 4 };
>    return baz(x);
> }
> 
> int bar(void)
> {
>    static const char x[] = { 1, 2, 3, 4 };
>    return baz(x);
> }
> --->8---
> 

I don't know where this assembly come from, but using gcc -O2, I'm getting the very same output for both functions for i386, x86_64, ppc and arm.

> Disassembly of section .text:
> 
> 00000000 <foo>:
>   0:   e52de004        push    {lr}            ; (str lr, [sp, #-4]!)
>   4:   e59f301c        ldr     r3, [pc, #28]   ; 28 <foo+0x28>
>   8:   e24dd00c        sub     sp, sp, #12
>   c:   e5933000        ldr     r3, [r3]
>  10:   e28d0008        add     r0, sp, #8
>  14:   e5203004        str     r3, [r0, #-4]!
>  18:   ebfffffe        bl      0 <baz>
>  1c:   e28dd00c        add     sp, sp, #12
>  20:   e49de004        pop     {lr}            ; (ldr lr, [sp], #4)
>  24:   e12fff1e        bx      lr
>  28:   00000000        .word   0x00000000
> 
> 0000002c <bar>:
>  2c:   e92d4008        push    {r3, lr}
>  30:   e59f0008        ldr     r0, [pc, #8]    ; 40 <bar+0x14>
>  34:   ebfffffe        bl      0 <baz>
>  38:   e8bd4008        pop     {r3, lr}
>  3c:   e12fff1e        bx      lr
>  40:   00000004        .word   0x00000004



-- Jean-Daniel