Why Linn DS is the best way to play CDs
Where does a company like Linn Products add value in a digital audio source product? It is not in the reading of ones and zeroes from the disk – there are many cheap, mass produced products that can serve this function very well.
Where Linn Products excels is in producing the best possible audio from the digital data. This is the one and only goal of the DS product range; this apparent limitation in reality means that the products are highly focused and deliver an unprecedented level of audio performance. The Linn DS products have been developed without any of the compromises that are inevitable in more ‘integrated’ products.
The function of a CD playback can be split into two basic elements
- Reading the data from the disk
- Converting the data to audio
Reading the data from the disk
The data on the disk is digital. Therefore the retrieval process can be perfect (considering that when you load software from a data CD it would not work if there were errors). In a traditional CD player the data is read at the same time as the audio is being reproduced. There is no technical benefit in this time linkage; indeed it has the disadvantage that if a read error occurs there is no time to attempt a re-read.
When a CD player encounters corrupt data it will attempt to mask the problem by extrapolating the good data, when a CD is ripped to computer disk there is a clever system of re-reading of difficult sections of the disk to ensure no corrupt data is stored to hard disk (this is why some CDs take a lot longer to rip).
In the Linn DS system the data is read to computer memory (hard-drive), this process can be verified as error-free by the use of checksums.
The transfer of data from the hard-drive to the Linn DS is over Ethernet using TCP-IP. This protocol ensures the integrity of the data delivered to the DACs.
Extensive end-to-end testing has verified that the data at the Linn DS is precisely the same data as on the original CD.
Converting the data to audio
Having ensured that the data is correct, the element that governs the quality of the audio is the digital to analog converter (DAC). There are a number of factors that influence the DAC performance:
- Noise sources
- Up-sampling and Quality of the DAC chip
- Topology of circuit
In a CD player there are inevitably moving parts. Even the highest quality CD mechanism produces noise, not just acoustic noise but electrical and magnetic noise.
In the listening room the acoustic noise is apparent to the user; however it is the electrical and magnetic noise that can be having a detrimental effect on the DAC circuit which may be the more significant element.
It is worth noting that this problem also exists for similar products with integral hard disk drives.
In the DS products this noise source is absent.
Linn DS has a similar clock architecture to Linn’s CD players. The audio clock is generated by an ultra low jitter oscillator located close to the DAC chips. This clock is used for the DACs and also provides the master timing reference to the entire digital path.
It is worth noting that this architecture where the data is ‘pulled’ from the network using the clock at the DAC is inherently better than systems where the data is transported by SPDIF or USB links, where the DAC has to ‘lock’ its clock to that at the sending end.
Up-sampling and Quality of the DAC chip
At Linn Products there is an on-going research project in which DAC chips are evaluated. For each candidate DAC chip a custom evaluation board is developed. These are subject to a suite of objective tests that we have developed for DAC assessment. The final arbitration for all these trial systems is the listening test.
On the basis of this research the Wolfson WM8741 has been selected for the Klimax DS.
In the Linn DS products we are using a custom up-sampling system that has been developed as a part of Linn’s DAC evaluation project. There are three advantages to this approach:
1. Choice of custom filter shape – again extensive listening tests were used.
2. We have far greater processing power available for up-sampling within an FPGA than is available from the built-in up-samplers in the DAC chips.
3. Electrical noise from the up-sampling process is located in the digital circuit (not in the sensitive analog part).
For more details on this subject you can read the Up-Sampling article.
Topology of circuit
The design and layout of the DAC circuit has a considerable impact on its performance. An area where the Klimax DS improves on previous Linn designs is the reduction of ground currents. This area is often overlooked by engineers who assume zero impedance ground. In reality there is always some resistance and inductance in all ground connections. It follows that any ground current (such as return current from digital signals) will inevitably cause a noise potential.
This is particularly damaging if the noise source is coherent with the audio signal – as is the case with the serial audio data signal.