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<chapter id="chapter-other-ntoone"
xreflabel="Writing a N-to-1 Element or Muxer">
<title>Writing a N-to-1 Element or Muxer</title>
N-to-1 elements have been previously mentioned and discussed in both
<xref linkend="chapter-advanced-request"/> and in
<xref linkend="chapter-scheduling"/>. The main noteworthy thing
about N-to-1 elements is that each pad is push-based in its own thread,
and the N-to-1 element synchronizes those streams by
expected-timestamp-based logic. This means it lets all streams wait
except for the one that provides the earliest next-expected timestamp.
When that stream has passed one buffer, the next
earliest-expected-timestamp is calculated, and we start back where we
were, until all streams have reached EOS. There is a helper base class,
called <classname>GstCollectPads</classname>, that will help you to do
Note, however, that this helper class will only help you with grabbing
a buffer from each input and giving you the one with earliest timestamp.
If you need anything more difficult, such as "don't-grab-a-new-buffer
until a given timestamp" or something like that, you'll need to do this
Note: I'd like to have something like this in the final text, but since
the code below doesn't work and this is all 0.8'y, I commented it for now.
<sect1 id="section-muxer-negotiation" xreflabel="Negotiation">
Most container formats will have a fair amount of issues with
changing content on an elementary stream. Therefore, you should
not allow caps to be changed once you've started using data from
them. The easiest way to achieve this is by using explicit caps,
which have been explained before. However, we're going to use them
in a slightly different way then what you're used to, having the
core do all the work for us.
The idea is that, as long as the stream/file headers have not been
written yet and no data has been processed yet, a stream is allowed
to renegotiate. After that point, the caps should be fixed, because
we can only use a stream once. Caps may then only change within an
allowed range (think MPEG, where changes in FPS are allowed), or
sometimes not at all (such as AVI audio). In order to do that, we
will, after data retrieval and header writing, set an explicit caps
on each sink pad, that is the minimal caps describing the properties
of the format that may not change. As an example, for MPEG audio
inside an MPEG system stream, this would mean a wide caps of
audio/mpeg with mpegversion=1 and layer=[1,2]. For the same audio type
in MPEG, though, the samplerate, bitrate, layer and number of channels
would become static, too. Since the (request) pads will be removed
when the stream ends, the static caps will cease to exist too, then.
While the explicit caps exist, the <function>_link ()</function>-
function will not be called, since the core will do all necessary
checks for us. Note that the property of using explicit caps should
be added along with the actual explicit caps, not any earlier.
Below here follows the simple example of an AVI muxer's audio caps
negotiation. The <function>_link ()</function>-function is fairly
normal, but the <function>-Loop ()</function>-function does some of
the tricks mentioned above. There is no <function>_getcaps ()</function>-
function since the pad template contains all that information already
(not shown).
static GstPadLinkReturn
gst_avi_mux_audio_link (GstPad *pad,
const GstCaps *caps)
GstAviMux *mux = GST_AVI_MUX (gst_pad_get_parent (pad));
GstStructure *str = gst_caps_get_structure (caps, 0);
const gchar *media = gst_structure_get_name (str);
if (!strcmp (str, "audio/mpeg")) {
/* get version, make sure it's 1, get layer, make sure it's 1-3,
* then create the 2-byte audio tag (0x0055) and fill an audio
* stream structure (strh/strf). */
} else if !strcmp (str, "audio/x-raw")) {
/* See above, but now with the raw audio tag (0x0001). */
} else [..]
static void
gst_avi_mux_loop (GstElement *element)
GstAviMux *mux = GST_AVI_MUX (element);
/* As we get here, we should have written the header if we hadn't done
* that before yet, and we're supposed to have an internal buffer from
* each pad, also from the audio one. So here, we check again whether
* this is the first run and if so, we set static caps. */
if (mux->first_cycle) {
const GList *padlist = gst_element_get_pad_list (element);
GList *item;
for (item = padlist; item != NULL; item = item->next) {
GstPad *pad = item->data;
GstCaps *caps;
if (!GST_PAD_IS_SINK (pad))
/* set static caps here */
if (!strncmp (gst_pad_get_name (pad), "audio_", 6)) {
/* the strf is the struct you filled in the _link () function. */
switch (strf->format) {
case 0x0055: /* mp3 */
caps = gst_caps_new_simple ("audio/mpeg",
"mpegversion", G_TYPE_INT, 1,
"layer", G_TYPE_INT, 3,
"bitrate", G_TYPE_INT, strf->av_bps,
"rate", G_TYPE_INT, strf->rate,
"channels", G_TYPE_INT, strf->channels,
case 0x0001: /* pcm */
caps = gst_caps_new_simple ("audio/x-raw",
} else if (!strncmp (gst_pad_get_name (pad), "video_", 6)) {
} else {
g_warning ("oi!");
/* set static caps */
gst_pad_use_explicit_caps (pad);
gst_pad_set_explicit_caps (pad, caps);
/* Next runs will never be the first again. */
mux->first_cycle = FALSE;
Note that there are other ways to achieve that, which might be useful
for more complex cases. This will do for the simple cases, though.
This method is provided to simplify negotiation and renegotiation in
muxers, it is not a complete solution, nor is it a pretty one.