Audio over Fiber: Sample Rate, Bit Depth and Real Sound Quality

First separate the sound signal from the digital samples

The sample rate and bit depth of an audio optical transceiver answer, respectively, how often the input voltage is read and how many bits represent each reading. They help in understanding the device, but they cannot by themselves prove the sound quality of the entire link. A digital optical transceiver used to extend analog line audio first turns the audio voltage into samples, then transmits them to the far end for reconstruction; a native digital audio interface already receives a digital bitstream and may not perform another analog-to-digital conversion.

The line output of a player or mixing console passes through input circuitry, filtering and an analog-to-digital converter ADC to form digital samples. The device organizes the samples of each channel into a transmission bitstream, sends them through the optical transmitter and fiber to the receiving end, and then a digital-to-analog converter DAC and reconstruction filtering restore the analog voltage. Each stage has its own limits; even if optical transmission is complete, it does not mean the input, conversion and output are all error-free.

Sample rate is time density, and cannot be written as sampling precision

Sample rate is expressed as how many samples per second, for example 48 kHz means forty-eight thousand times per second. It determines the spacing of samples on the time axis. For a band-limited signal, the sampling frequency must be higher than twice the highest frequency retained; in engineering, space must also be left for the filter transition band, and exactly twice must not be treated as a guarantee that actual equipment can unconditionally recover the boundary frequency.

Anti-aliasing filtering is used to limit out-of-band content before sampling. Without suitable filtering, higher frequencies may fold into erroneous components within the band after sampling, and this cannot be corrected later simply by increasing playback volume. 44.1 kHz, 48 kHz and 96 kHz are different sampling configurations and should not be directly ranked as levels of good sound; a project must look at the actual bandwidth, processing chain and the configurations supported by all devices.

Bit depth is the amplitude scale, and transmission bitrate is calculated separately

Bit depth is expressed in bits, for example 16 bit or 24 bit, and describes the number of binary digits used for each sample. Ideally, the more bits, the finer the amplitude scale that can be represented. Quantization is the process of assigning a continuous voltage to these finite levels, which creates quantization error; it does not mean the input is truly noise-free, and it will not automatically repair incorrectly wired lines or sound that has already been distorted by an amplifier.

Transmission bitrate is expressed in bit/s. If we assume two mono channels both use 48 kHz, 24 bit and are uncompressed, the net sample data rate alone is 48000 times 24 times 2, giving 2.304 Mbps; this does not include frame headers, synchronization, line coding or other services. This number is a teaching calculation, not the optical port rate of any particular optical transceiver, and it cannot be used to infer that any transmitter and receiver can interoperate.

A 24-bit label cannot replace a complete-device noise measurement

For an ideal quantizer under specified conditions such as a full-scale sine wave, the quantization signal-to-noise ratio increases by about 6 dB per bit as the number of bits grows. This theoretical relationship cannot be rewritten to mean that a 24-bit complete device must have 144 dB of dynamic range. Real equipment also has noise and distortion from the analog front end, power supply, clock, conversion circuitry and output section, so the complete-device test conditions and measurement results should be checked.

Signal-to-noise ratio describes the size of the signal relative to noise, while dynamic range and total harmonic distortion have their own definitions and measurement methods. Whether weighting is specified, the input level, test frequency, output load and measurement bandwidth all affect comparison. Confusing the three metrics, or substituting a chip’s theoretical limit for the complete-device specification, can make a purchase look numerically high but fail acceptance.

Input clipping and the noise floor enter the digital samples

When the input exceeds the allowed range, the peaks of the waveform are flattened, which is called clipping. The ADC records the clipped voltage, and even if the receiving end accurately restores these samples, it cannot reconstruct the original peak shape. Conversely, if the input is too small and the later gain is very high, the front-end noise will also be amplified. Allocating gain reasonably solves these on-site problems better than blindly choosing higher sampling parameters.

Microphones, line outputs and amplifier speaker outputs are not the same level. An ordinary line input cannot directly withstand an amplifier power output; whether a microphone can be plugged in directly depends on the preamp gain and power supply. Digital optical transmission itself does not automatically provide noise reduction, compression, echo cancellation or volume equalization; these processing functions must be checked for the specific product and cannot be assumed from the word “digital”.

Before raising parameters, check both ends and the system configuration

A higher sample rate increases the number of samples per unit time, a higher bit depth increases the amount of data per sample, and more channels also increase the total demand. Comparison is meaningful only when the transmitter, receiver, internal processing and downstream all support the corresponding configuration. A fixed-sampling product may not allow user adjustment; a native digital input also requires checking receiver lock, clock relationships and whether sample rate conversion is supported.

Uncompressed sample transmission is different from line coding, and not all optical transceivers can be said to use AAC or other lossy audio compression. When a project requires bit-identical transmission, it should specify which digital interface and which valid data are involved; when a project requires analog sound quality, it should test from analog input to analog output, and not replace frequency response and distortion checks with a passing optical link bit error test.

Understand Feichang Technology product fields in the correct units

Feichang Technology FCA-A4-L documentation lists information such as RCA input and AD/DA bit width; FCA-A8-H lists XLR and phantom power. Interface differences affect how source equipment is connected, and the power supply capability of one product cannot be generalized to another. When the historical overview and parameter table on a page are written differently, the manufacturer should be asked to confirm the sample frequency, input range and test conditions of the ordered version.

When reading documentation, bit corresponds to bit width, kHz corresponds to sample frequency, and Mbps corresponds to bitrate; dB and dBm are also not the same thing. Purchase confirmation should record the actual number of mono channels, direction, frequency response, noise and distortion conditions. Chip documentation is used to explain mechanisms and does not mean that Feichang Technology uses that chip, still less should chip specifications be written as guarantees for a Feichang Technology complete device.

For acceptance, compare the entire audio chain under the same conditions

First use a known sound source within the allowed input range to establish a short-link baseline, and record the source level, gain, sampling configuration and downstream load. When quantitative comparison is needed, use suitable audio measurement instruments to check frequency response, noise and distortion; a phone recording can document a phenomenon but is not sufficient to replace specification testing under specified conditions. Before listening, lower the amplifier volume and gradually adjust it to a suitable range.

Then connect the formal optical cable and all planned channels, and observe continuous operation, fiber breakage and recovery after restart. Latency must clearly state the input and output measurement points, including the digital mixing, effects and monitoring processing actually used. What is delivered is a reproducible configuration and measurement conclusion, not generalized conclusions such as “the higher the sample rate, the better” or “fiber is absolutely lossless”.

Translated from the Chinese technical article. Source first published: 2024-12-19. Source last revised: 2026-10-15T19:44:23.840277+08:00. Read the Chinese source