aacenc_tns: use 4 bits for short windows

With only 7 coefficients per short window at most the extra precision
makes a difference and seems to reduce crackling and stddev even
further.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/aacenc_tns.c

@@ -35,7 +35,7 @@
 #define TNS_Q_BITS 4
 
 /* Coefficient resolution in short windows */
-#define TNS_Q_BITS_IS8 3
+#define TNS_Q_BITS_IS8 4
 
 /* We really need the bits we save here elsewhere */
 #define TNS_ENABLE_COEF_COMPRESSION

tests/fate/aac.mak

@@ -182,7 +182,7 @@ fate-aac-tns-encode: CMD = enc_dec_pcm adts wav s16le $(TARGET_SAMPLES)/audio-re
 fate-aac-tns-encode: CMP = stddev
 fate-aac-tns-encode: REF = $(SAMPLES)/audio-reference/luckynight_2ch_44kHz_s16.wav
 fate-aac-tns-encode: CMP_SHIFT = -4096
-fate-aac-tns-encode: CMP_TARGET = 818
+fate-aac-tns-encode: CMP_TARGET = 817
 fate-aac-tns-encode: FUZZ = 6
 fate-aac-tns-encode: SIZE_TOLERANCE = 3560