What do you mean by "48kHZ improved the sound very much on C600N"?
C600N has aptX HD capability, but it's not advertised on Sony website.
Using Mac os X Audio and Midi Utility, you may see 3 choices for C600N Audio Output:
- 8000Hz which is used for phone in/out
- 441000Hz which is either for AAC/aptX/SBC
- 48000Hz which is probably aptX enhanced as specified by RFC 7310 (24bits / 48kHZ), also known as aptX HD IMHO.
3. Standard apt-X and Enhanced apt-X Codecs
Standard apt-X and Enhanced apt-X are proprietary audio coding
algorithms, which can be licensed from CSR plc. and are widely
deployed in a variety of audio processing equipment. For commercial
reasons, the detailed internal operations of these algorithms are not
described in standards or reference documents. However, the data
interfaces to implementations of these algorithms are very simple and
allow easy RTP packetization of data coded with the algorithms
without detailed knowledge of the actual coded audio stream syntax.
Both the Standard apt-X and Enhanced apt-X coding algorithms are
based on Adaptive Differential Pulse Code Modulation principles.
They produce a constant coded bit rate that is scaled according to
the sample frequency of the uncoded audio. This constant rate is 1/4
of the bit rate of the uncoded audio, irrespective of the resolution
(number of bits) used to represent an uncoded audio sample. For
example, a 1.536-Mbit/s stereo audio stream composed of two channels
of 16-bit Pulse Code Modulated (PCM) audio that is sampled at a
frequency of 48 kHz is encoded at 384 kbit/s.
Standard apt-X and Enhanced apt-X do not enforce a coded frame
structure, and the coded data forms a continuous coded sample stream
with each coded sample capable of regenerating four PCM samples when
decoded. The Standard apt-X algorithm encodes four successive 16-bit
PCM samples from each audio channel into a single 16-bit coded sample
per audio channel. The Enhanced apt-X algorithm encodes four
successive 16-bit or 24-bit PCM samples from each audio channel and
respectively produces a single 16-bit or 24-bit coded sample per
channel. The same RTP packetization rules apply for each of these
algorithmic variations.
https://tools.ietf.org/html/rfc7310